[clang] [Clang][OpenMP] Emit unsupported directive error (PR #70233)

[Shilei Tian via cfe-commits]

shiltian wrote:

Can you add a small test to check the error message is correctly emitted? You can refer to those diagnosis tests under `clang/test/OpenMP`.

https://github.com/llvm/llvm-project/pull/70233