[clang] [clang][deps] Load module map file from PCM (PR #66389)

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

Closing. I want to achieve this by fixing the underlying issues:
* Speeding up `SLocEntry` deserialization (https://github.com/llvm/llvm-project/pull/66962, https://github.com/llvm/llvm-project/pull/66966).
* Either adding caching to `DependencyScanningWorkerFilesystem::getRealPath()`, or by sharing `FileManager` across module builds and TU scans (should be sound after removing all uses of `FileEntry::getName()`, `FileEntry::getLastRef()`).

https://github.com/llvm/llvm-project/pull/66389