[libcxx] [llvm] [libc++][NFC] Make AssertionInfoMatcher::CheckMessageMatches Stricter (PR #77721)

[Louis Dionne via llvm-commits]

https://github.com/ldionne approved this pull request.

I am fine with this change if it passes CI (and without the unrelated changes). I thought we took advantage of this "hand wavyness" in more places, but if we don't and the CI is happy, I think this is an improvement.

https://github.com/llvm/llvm-project/pull/77721