[flang][OpenMP] Compile proper omp_lib.mod from the openmp/src/include sources (#80874)

This PR changes the build system to use use the sources for the module `omp_lib` and the `omp_lib.h` include file from the `openmp` runtime project and not from a separate copy of these files. This will greatly reduce potential for inconsistencies when adding features to the OpenMP runtime implementation. When the OpenMP subproject is not configured, this PR also disables the corresponding LIT tests with a "REQUIRES" directive at the beginning of the OpenMP test files. --------- Co-authored-by: Valentin Clement (バレンタイン クレメン) <clementval@gmail.com>

Author: mjklemm