[Polly] [NFC] [ScopDetection] Make `polly-only-func` perform regex scop name match.

Summary:

- We were using `.count` in `StringRef`, which matches substrings.
- We may want to use this for equality as well.
- Generalise this, so allow regexes as a parameter to `polly-only-func`.

Differential Revision: https://reviews.llvm.org/D35728

llvm-svn: 308875