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/automake.sw.multi.approved.txt

@@ -100,6 +100,8 @@
 :test-result: PASS CAPTURE can deal with complex expressions involving commas
 :test-result: PASS CAPTURE parses string and character constants
 :test-result: PASS Capture and info messages
+:test-result: FAIL Captures do not leave block with an exception
+:test-result: FAIL Captures outlive section end
 :test-result: PASS CaseInsensitiveEqualsTo is case insensitive
 :test-result: PASS CaseInsensitiveLess is case insensitive
 :test-result: PASS Character pretty printing
@@ -224,6 +226,9 @@
 :test-result: PASS Scenario: Do that thing with the thing
 :test-result: PASS Scenario: This is a really long scenario name to see how the list command deals with wrapping
 :test-result: PASS Scenario: Vector resizing affects size and capacity
+:test-result: FAIL Scoped message applies to all assertions in scope
+:test-result: FAIL Scoped messages do not leave block with an exception
+:test-result: FAIL Scoped messages outlive section end
 :test-result: FAIL Sends stuff to stdout and stderr
 :test-result: PASS Some simple comparisons between doubles
 :test-result: FAIL Standard output from all sections is reported