[clang] [NFC][Clang][OpenMP] Add helper functions/utils for finding/comparing attach base-ptrs. (PR #155625)

[Abhinav Gaba via cfe-commits]

abhinavgaba wrote:

@alexey-bataev, this is the NFC PR that we were talking about this morning. The functional PR that uses these (which I'll update once this is merged, to reduce the code size, is #153683).

https://github.com/llvm/llvm-project/pull/155625