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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

This indicates that `LLVM_TARGET_TRIPLE` is not defined, which seems quite unusual. How does the runtime no know what target it is building for?

You were included as reviewer in the original PR #137828 without participarting in it. LLVM's workflow explicitly mentions [post-commit reviews](https://llvm.org/docs/CodeReview.html#can-code-be-reviewed-after-it-is-committed).

Were have been updating our CI builders and are already building patches on top of it, so a revert would come with some friction to us.

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