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

[Alexey Bataev via cfe-commits]

alexey-bataev wrote:

Having these functions without actual use is meaningless; better to split the patch by the supported types or other criteria, but with the actual uses

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