[clang][modules] Track affecting modules

When compiling a module, its semantics and Clang's behavior are affected by other modules. These modules are typically the **imported** ones. However, during implicit build, some modules end up being compiled and read without being actually imported. This patch starts tracking such modules and serializing them into `.pcm` files. This enables the dependency scanner to construct explicit compilations that mimic implicit build.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D132430