[flang][NFC] Refactor MODULE definitions to not accidentally leak symbols (#80833)

This PR continues the work started with PR #79006, by setting visibility
in MODULES to PRIVATE by default and explicitly exporting only the
desired symbols. `omp_lib` needs more work, as it should maybe be
compiled from `omp_lib.f90` in
`openmp/runtime/src/incluce/omp_lib.f90.var` instead of simply using an
INCLUDE for `omp_lib.h`