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

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

To better understand the motivation, I have two questions:
* Why do you give Clang module maps describing modules that are already described in PCM files? Unless the module map describes some other top-level module that's yet to be built, the information it provides is redundant. Is this impossible to detect in your build system?
* Why do you need to wait for 64-bit source locations to be the default upstream? Is the concern that it's not a well-tested configuration right now and you don't want to shoulder the burden of qualifying that mode?

This patch looks good to me, but I think it'd be worth pursuing making this the default behavior, i.e. investigating the test failures and at least understanding what exactly is going on.

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