[openmp] [clang] [OpenMP] Directly use user's grid and block size in kernel language mode (PR #70612)

[Johannes Doerfert via cfe-commits]

https://github.com/jdoerfert approved this pull request.

LG, see the nit.

Also, add a runtime test with bare and verify we stick with values we otherwise would not, e.g., 10000 teams and 1024 threads.

https://github.com/llvm/llvm-project/pull/70612