[clang] [flang] [clang][OpenMP] Diagnose invalid allocator in `#pragma omp allocate`; avoid null deref (PR #158146)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

Thank you for you PR, there are a bunch of small issues and some questions that need replies.

https://github.com/llvm/llvm-project/pull/158146