[clang] [Modules] Add -cc1 -flate-module-map-file to load module maps after PCMs (PR #88893)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

There's quite a few failures:
https://gist.github.com/ilya-biryukov/380d84dfe53f839f449231eb9a2dd46c

The logic that sets up module structure from module maps before starting to load PCMs definitely needs to be reworked (and I'm not sure if it can be in the first place). I believe we get away with it internally because we always pass either a top-level PCM or a top-level module map for every top-level dependency of our compilation. Therefore, our loading of PCMs ends up being self-contained, which is not always the case. This probably can't be fixed.

Btw, #89005 might make this change unnecessary for us as it should also unbreak our existing builds.

https://github.com/llvm/llvm-project/pull/88893