[clang] [flang] [llvm] [openmp] [Flang] Move builtin .mod generation into runtimes (Reapply #137828) (PR #171515)

[Petr Hosek via llvm-commits]

petrhosek wrote:

Would it be possible to break up this change into multiple PRs? Specifically, I believe the Clang driver changes, moving of `GetToolchainDirs.cmake` to `cmake/Modules`, and changes to `runtimes/CMakeLists.txt` would ideally be split into separate PRs and merged first to reduce the size of this change which is quite large to review as is.

https://github.com/llvm/llvm-project/pull/171515