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

[Jan Patrick Lehr via llvm-commits]

jplehr wrote:

I have updated the PR to add 4 different labels. The file list for the `clang` and `flang` parts are specified directly whereas it felt easier to use `Match Objects` (https://github.com/actions/labeler) to include everything in the `openmp` directory except the ones in `libomptarget` and the other way around for the two runtime labels.
If this is acceptable, I'll create the labels (should I rename `openmp` to anything or leave it for now?) and merge the PR.

I have tested the rules for `openmp:libomp` and `openmp:libomptarget` locally and they seem to work.

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