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

[Krish Gupta via cfe-commits]

KrxGu wrote:

@shafik reverted the comments, brought back `k`, have tested and built again everything working fine.

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