[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.

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Co-authored-by: Valentin Clement (バレンタイン クレメン) <clementval@gmail.com>