[llvm] [FileCheck] Allow -check-prefix to take multiple prefixes (PR #178187)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 approved this pull request.

Documentation build needs to be fixed, but otherwise LGTM.

We incur basically zero complexity in `FileCheck` to support this and I have wished for this in the past, so seems reasonable enough to me.

https://github.com/llvm/llvm-project/pull/178187