[clang] Fix `pragma-comment.c` test failing on false match (PR #145455)

[Aaron Ballman via cfe-commits]

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

LGTM! Thank you!

I'll land the changes on your behalf once precommit CI comes back green.

https://github.com/llvm/llvm-project/pull/145455