[clang] [flang] [llvm] [openmp] [Flang] Move builtin .mod generation into runtimes (Reapply #137828) (PR #169638)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

After working around the CMAKE_Fortran_PREPROCESS_SOURCE failure, [openmp-offload-amdgpu-clang-flang](https://lab.llvm.org/staging/#/builders/105) is [failing](https://lab.llvm.org/staging/#/builders/105/builds/37419) with a different problem: The builder is running Fortran+OpenMP offload tests on amdgpu-amd-amdhsa. For these to compile, omp_lib.mod and the builtin modules must be compiled for amdgpu-amd-amdhsa as well.

1. The builtin modules are compiled by just adding/not removing [LLVM_ENABLE_RUNTIMES=flang-rt](https://github.com/llvm/llvm-zorg/pull/666).

2. `omp_lib.mod` unfortunately involves more changes. openmp skips [`add_subdirectory(runtime)` for amdgpu/nvptx](https://github.com/llvm/llvm-project/blob/main/openmp/CMakeLists.txt#L138-L139) which contains the module build instructions. Some [restructure](https://github.com/llvm/llvm-project/pull/169638/commits/7732534216ede9d5cff2fcb1868479cb69a33618) was needed to compile the Fortran modules for offload triples as well, mostly moving definitions out of the `runtime/` directory into a new `module/` directory for building omp_lib.mod for host and device.

https://github.com/llvm/llvm-project/pull/169638