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

[Richard Smith via cfe-commits]

zygoloid wrote:

Have you tried changing the behavior of the existing `-fmodule-map-file=` to load the file after we load module files? If so, what kinds of things do you see breaking or changing? If we can avoid it, I think it would be better to only have a single mode here, and loading the module files first seems to make a lot of sense given that they can make it unnecessary to load the module map files at all.

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