[clang] Add CHECK-LABEL to avoid source tree path sensitivity in test (PR #112461)

[Justin Bogner via cfe-commits]

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

Could also consider checking for `ret void` instead, as that's more flexible if someone were to add another test case later in the file. This seems fine too though.

https://github.com/llvm/llvm-project/pull/112461