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

Hello!

This PR fixes #63871. Clang should no longer crash and instead emits an
error message.

Below is an example of the new error message:

```
~/dev/fork-llvm-project omp_dispatch_unimpl
❯ ./install/bin/clang -fopenmp  -c -emit-llvm -Xclang -disable-llvm-passes test.c
test.c:6:5: error: cannot compile this OpenMP dispatch directive yet
    6 |     #pragma omp dispatch
      |     ^~~~~~~~~~~~~~~~~~~~
1 error generated.
```