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

[Petr Hosek via llvm-commits]

petrhosek wrote:

This broke the runtimes build on Windows when targeting a non-Windows target with the following error:
```
CMake Error at C:/b/s/w/ir/x/w/llvm-llvm-project/cmake/Modules/GetToolchainDirs.cmake:76 (string):
  string sub-command REPLACE requires at least four arguments.
Call Stack (most recent call first):
  C:/b/s/w/ir/x/w/llvm-llvm-project/cmake/Modules/GetToolchainDirs.cmake:42 (get_toolchain_arch_dirname)
  CMakeLists.txt:459 (get_toolchain_library_subdir)
```
Would it be possible to revert the change while this is being investigated?

Since this makes significant changes to the runtimes build that affects non-Flang users, I'd also appreciate to be included as a reviewer.

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