[flang] [llvm] [Frontend][OpenMP] introduce OMP.h header file, use it instead of OMP… (PR #84188)

[Michael Klemm via llvm-commits]

mjklemm wrote:

Nit: Would it be possible to rename both files to not only have `OMP` in the basename?  I know that it does not conflict with `omp.h`, which is the header file of the OpenMP API itself, but having a clearer name for `OMP.h.inc` and `OMP.h` would help avoid potential conflicts with `omp.h` on a case-insensitive file system.

https://github.com/llvm/llvm-project/pull/84188