[Clang][OpenMP] Emit unsupported directive error

[rkchang]

Hello!

This PR fixes #63871. Clang should no longer crash and instead emits an error message.
@shiltian is this what you had in mind for an error message? I think I followed everything in [1] but I did not add a unit test as this behavior should change in the future when dispatch is implemented. Also I did not find any existing unit tests that test for behavior like this.

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.

Please let me know if there's anything you'd like me to change.

PS. This is my first open source contribution so if there's any etiquette or instructions I missed please let me know.

[1] https://llvm.org/docs/Contributing.html

[shiltian]

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.

[rkchang]

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%)

[shiltian]

LG with a nit