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

KrxGu wants to merge 6 commits intollvm:mainllvm/llvm-project:mainfrom

KrxGu:clang-omp-allocate-npe-fixKrxGu/llvm-project:clang-omp-allocate-npe-fixCopy head branch name to clipboard