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

[Raymond Chang via cfe-commits]

rkchang wrote:

Added a test case. Thanks for the pointer! Here's the result:

```
~/dev/fork-llvm-project omp_dispatch_unimpl
❯ llvm-lit -vv clang/test/OpenMP/dispatch_unsupported.c
llvm-lit: /home/rkchang/dev/fork-llvm-project/llvm/utils/lit/lit/llvm/config.py:488: note: using clang: /home/rkchang/dev/fork-llvm-project/build/bin/clang
-- Testing: 1 tests, 1 workers --
PASS: Clang :: OpenMP/dispatch_unsupported.c (1 of 1)

Testing Time: 0.07s

Total Discovered Tests: 1
  Passed: 1 (100.00%)
```

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