[clang] [clang][modules] Separate parsing of modulemaps (PR #119740)

[Volodymyr Sapsai via cfe-commits]

https://github.com/vsapsai commented:

Overall the change makes sense and now `ModuleMap` works more like semantic analysis after the module map parsing.

Didn't really delve into the parsing code and didn't compare the implementations side-by-side. Relying on the tests and expect most of the code to be just moved. Can review it more thoroughly if you suspect extra complexity there.

Do we need to carry the current working directory somewhere in `ModuleMapFile` to help with resolving the file names?

https://github.com/llvm/llvm-project/pull/119740