[llvm] [Github][OpenMP] Adding rule for OpenMP label (PR #65331)

[Jan Patrick Lehr via llvm-commits]

jplehr wrote:

Is the overall feeling that we should have separate labels `clang-openmp`, `flang-openmp` and `openmp`?
The latter would cover everything inside the `openmp` folder, i.e., the runtime, whereas the other two are for the "frontend" parts of the project?

https://github.com/llvm/llvm-project/pull/65331