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

[Will Hawkins via llvm-commits]

hawkinsw wrote:

Pinging @var-const given his work on https://github.com/llvm/llvm-project/pull/77883. I will wait for his "go" before moving forward.

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