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

[Eugene Epshteyn via llvm-commits]

eugeneepshteyn wrote:

@Meinersbur , with a potentially disruptive change like this, it would have been helpful to send a heads-up to flang discourse, describing what's about to be merged, what are potential implications and how to handle them. And also add the same notice as a comment to this PR, so people could find it and deal with the fallout.

https://github.com/llvm/llvm-project/pull/137828