[OpenMP] Install the Fortran module files with libomp-mod (#227116)

Summary:
The `libomp-mod` install component only contains `omp_lib.h`. The module
files are only installed incidentally as part of the Fortran runtime or
directory copy. Just do this directly. These were intentinoally dropped
in https://github.com/llvm/llvm-project/pull/171515, in favor of the
directory copy. However, this means these are ignored when done through
the specific component target.

Shouldn't affect plain installs.

GitOrigin-RevId: c7b2f3191aab2d563d9146d828a195df6103292f