[clang][modules][deps] Parent module maps are affecting

Currently, the algorithm for gathering affecting module maps includes only those defining modules that include some headers. This is not entirely correct, though. Some module maps might be "importing" module maps for `extern` submodules. Such parent module maps are affecting - they do change semantics of the compilation. This patch adds parent module maps into the set of affecting module maps.

Depends on D137197.

Reviewed By: Bigcheese

Differential Revision: https://reviews.llvm.org/D137198