Regularize scoped message lifetime to only consider the object's scope

This means that:
1) Scoped messages are always removed at the end of their scope,
   even if the scope ended due to an exception.
2) Scoped messages outlive section end, if that section's scope is
   enclosed in their own.

Previously neither of these were true, which has led to a number
of surprising behaviour, where e.g. this:
```cpp
TEST_CASE() {
    try {
        INFO( "some info" );
        throw std::runtime_error( "ex" );
    } catch (std::exception const&) {}

    REQUIRE( false );
}
```
would print "some info" as the message for the assertion, while this:
```cpp
TEST_CASE() {
    INFO("Hello");
    SECTION("dummy") {}
    REQUIRE(false);
}
```
would not print out "Hello" as the message for the assertion.

This had an underlying reason, in that it was trying to helpfully
keep the messages around in case of unexpected exceptions, so that
code like this:
```cpp
TEST_CASE() {
    auto [input, expected] = GENERATE(...);
    CAPTURE(input);
    auto result = transform(input); // throws
    REQUIRE(result == expected);
}
```
would report the value of `input` when `transform` throws. However,
it was surprising in practice and was causing various issues around
handling of messages in other cases.

Closes #1759
Closes #2019
Closes #2959

tests/SelfTest/Baselines/xml.sw.approved.txt

@@ -683,9 +683,6 @@ Nor would this
   </TestCase>
   <TestCase name="#748 - captures with unexpected exceptions" tags="[!shouldfail][!throws][.][failing]" filename="tests/<exe-name>/UsageTests/Exception.tests.cpp" >
     <Section name="outside assertions" filename="tests/<exe-name>/UsageTests/Exception.tests.cpp" >
-      <Info filename="tests/<exe-name>/UsageTests/Exception.tests.cpp" >
-        answer := 42
-      </Info>
       <Expression success="false" filename="tests/<exe-name>/UsageTests/Exception.tests.cpp" >
         <Original>
           {Unknown expression after the reported line}
@@ -3090,6 +3087,48 @@ Approx( 1.23399996757507324 )
     </Section>
     <OverallResult success="true" skips="0"/>
   </TestCase>
+  <TestCase name="Captures do not leave block with an exception" tags="[.][capture][failing][messages]" filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+    <Info filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+      a := 1
+    </Info>
+    <Expression success="false" type="REQUIRE" filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+      <Original>
+        false
+      </Original>
+      <Expanded>
+        false
+      </Expanded>
+    </Expression>
+    <OverallResult success="false" skips="0"/>
+  </TestCase>
+  <TestCase name="Captures outlive section end" tags="[.][failing][info][messages]" filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+    <Section name="Dummy section" filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+      <Info filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+        a := 1
+      </Info>
+      <Expression success="true" type="CHECK" filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+        <Original>
+          true
+        </Original>
+        <Expanded>
+          true
+        </Expanded>
+      </Expression>
+      <OverallResults successes="1" failures="0" expectedFailures="0" skipped="false"/>
+    </Section>
+    <Info filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+      a := 1
+    </Info>
+    <Expression success="false" type="REQUIRE" filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+      <Original>
+        false
+      </Original>
+      <Expanded>
+        false
+      </Expanded>
+    </Expression>
+    <OverallResult success="false" skips="0"/>
+  </TestCase>
   <TestCase name="CaseInsensitiveEqualsTo is case insensitive" tags="[comparisons][string-case]" filename="tests/<exe-name>/IntrospectiveTests/Details.tests.cpp" >
     <Section name="Degenerate cases" filename="tests/<exe-name>/IntrospectiveTests/Details.tests.cpp" >
       <Expression success="true" type="REQUIRE" filename="tests/<exe-name>/IntrospectiveTests/Details.tests.cpp" >
@@ -12893,6 +12932,73 @@ All available test cases:
     </Section>
     <OverallResult success="true" skips="0"/>
   </TestCase>
+  <TestCase name="Scoped message applies to all assertions in scope" tags="[.][failing][info][messages]" filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+    <Info filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+      This will be reported multiple times
+    </Info>
+    <Expression success="false" type="CHECK" filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+      <Original>
+        false
+      </Original>
+      <Expanded>
+        false
+      </Expanded>
+    </Expression>
+    <Info filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+      This will be reported multiple times
+    </Info>
+    <Expression success="false" type="CHECK" filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+      <Original>
+        false
+      </Original>
+      <Expanded>
+        false
+      </Expanded>
+    </Expression>
+    <OverallResult success="false" skips="0"/>
+  </TestCase>
+  <TestCase name="Scoped messages do not leave block with an exception" tags="[.][failing][info][messages]" filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+    <Info filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+      Should be in scope at the end
+    </Info>
+    <Expression success="false" type="REQUIRE" filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+      <Original>
+        false
+      </Original>
+      <Expanded>
+        false
+      </Expanded>
+    </Expression>
+    <OverallResult success="false" skips="0"/>
+  </TestCase>
+  <TestCase name="Scoped messages outlive section end" tags="[.][failing][info][messages]" filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+    <Section name="Dummy section" filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+      <Info filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+        Should survive a section end
+      </Info>
+      <Expression success="true" type="CHECK" filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+        <Original>
+          true
+        </Original>
+        <Expanded>
+          true
+        </Expanded>
+      </Expression>
+      <OverallResults successes="1" failures="0" expectedFailures="0" skipped="false"/>
+    </Section>
+    <Info filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+      Should survive a section end
+    </Info>
+    <Expression success="false" type="REQUIRE" filename="tests/<exe-name>/UsageTests/Message.tests.cpp" >
+      <Original>
+        false
+      </Original>
+      <Expanded>
+        false
+      </Expanded>
+    </Expression>
+    <OverallResult success="false" skips="0"/>
+  </TestCase>
   <TestCase name="Sends stuff to stdout and stderr" tags="[.]" filename="tests/<exe-name>/UsageTests/Misc.tests.cpp" >
     <OverallResult success="false" skips="0">
       <StdOut>
@@ -22007,6 +22113,6 @@ Approx( -1.95996398454005449 )
     </Section>
     <OverallResult success="true" skips="0"/>
   </TestCase>
-  <OverallResults successes="2087" failures="151" expectedFailures="35" skips="12"/>
-  <OverallResultsCases successes="313" failures="90" expectedFailures="14" skips="6"/>
+  <OverallResults successes="2089" failures="157" expectedFailures="35" skips="12"/>
+  <OverallResultsCases successes="313" failures="95" expectedFailures="14" skips="6"/>
 </Catch2TestRun>