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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

@jhuber6 Based on the feedback in #137828, I made it easier to build the Fortran modules for nvptx and amdgpu targets. It is now possible to set both FLANG_RT_ENABLE_STATIC and FLANG_RT_ENABLE_SHARED to off, meaning no library is build, only the modules files. This is the default if the target is nvptx or amdgpu. However, this overrides the (experimental) default behavior of #131826. I could not get the gpu_sources to compile since it cannot find include files such as `#include <cassert>`. I could trick it to point to the libc++ versions, but I don't want other users have to do many steps. Please have a look specifically to [`9041b78` (#169638)](https://github.com/llvm/llvm-project/pull/169638/commits/9041b784e3c054266a9109c2698030ea4f2aad44)

In the spirit off adding `LLVM_ENABLE_RUNTIMES=flang-rt` automatically, we could also automatically add `LLVM_RUNTIME_TARGETS=default;amdgcn-amd-amdhsa` when the AMDGPU backend is enabled; respectively `LLVM_RUNTIME_TARGETS=default;nvptx64-nvidia-cuda` if `LLVM_TARGETS_TO_BUILD=NVPTX` is present. What do you think?

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