[openmp][flang] To add a cmake option to build OPENMP Fortran modules only without building the libomp lib (#195576)
PR #192687 Introduces a dependency to the Fortran intrinsic module when
building `openmp` Fortran modules, which requires a Flang enabled llvm
build in order to build `openmp` Fortran modules.
This PR is to add a cmake option `LIBOMP_FORTRAN_MODULES_ONLY` to allow
users to build 'openmp` Fortran modules only for Flang without building
the libomp runtime library.
This will allow libomp.a/so build still be independent from the Flang
build as before.
E.g. To build `openmp` Fortran modules only for Flang, one would need to
use the following options.
```
cmake \
...
-DLIBOMP_FORTRAN_MODULES_ONLY=ON \
-DLLVM_ENABLE_PROJECTS="flang" \
-DLLVM_ENABLE_RUNTIMES="openmp" \
...