[clang] [llvm] [FileCheck] Avoid matching IR keywords in source filenames (PR #213809)

[Erich Keane via cfe-commits]

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

That works for OpenACC.  Not sure about the other tests, could we do something similar instead? 

https://github.com/llvm/llvm-project/pull/213809