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/UsageTests/Message.tests.cpp

@@ -310,3 +310,53 @@ TEST_CASE( "INFO and UNSCOPED_INFO can stream multiple arguments",
                    << " parts." );
     FAIL( "Show infos!" );
 }
+
+TEST_CASE( "Scoped messages do not leave block with an exception", "[messages][info][.failing]" ) {
+    INFO( "Should be in scope at the end" );
+    { INFO( "This should go out of scope immediately" ); }
+
+    try {
+        INFO( "Should not be in scope at the end" );
+        throw std::runtime_error( "ex" );
+    } catch (std::exception const&) {}
+
+    REQUIRE( false );
+}
+
+TEST_CASE( "Captures do not leave block with an exception",
+           "[messages][capture][.failing]" ) {
+    int a = 1, b = 2, c = 3;
+
+    CAPTURE( a );
+    { CAPTURE( b ); }
+
+    try {
+        CAPTURE( c );
+        throw std::runtime_error( "ex" );
+    } catch ( std::exception const& ) {}
+
+    REQUIRE( false );
+}
+
+TEST_CASE( "Scoped messages outlive section end",
+           "[messages][info][.failing]" ) {
+    INFO( "Should survive a section end" );
+    SECTION( "Dummy section" ) { CHECK( true ); }
+
+    REQUIRE( false );
+}
+
+TEST_CASE( "Captures outlive section end", "[messages][info][.failing]" ) {
+    int a = 1;
+    CAPTURE( a );
+    SECTION( "Dummy section" ) { CHECK( true ); }
+
+    REQUIRE( false );
+}
+
+TEST_CASE( "Scoped message applies to all assertions in scope",
+           "[messages][info][.failing]" ) {
+    INFO( "This will be reported multiple times" );
+    CHECK( false );
+    CHECK( false );
+}