[clang][modules] Consider M affecting after mapping M.Private to M_Private

When Clang encounters `@import M.Private` during implicit build, it precompiles module `M` and looks through its submodules. If the `Private` submodule is not found, Clang assumes `@import M_Private`. In the dependency scanner, we don't capture the dependency on `M`, since it's not imported. It's an affecting module, though: compilation of the import statement will fail when implicit modules are disabled and `M` is not precompiled and explicitly provided. This patch fixes that.

Depends on D132430.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D132502