[clang][modules] Avoid re-exporting PCH imports on every later module import

We only want to make PCH imports visible once for the the TU, not
repeatedly after every subsequent import. This causes some incorrect
behaviour with submodule visibility, and causes us to get extra module
dependencies in the scanner. So far I have only seen obviously incorrect
behaviour when building with -fmodule-name to cause a submodule to be
textually included when using the PCH, though the old behaviour seems
wrong regardless.

rdar://107449644

Differential Revision: https://reviews.llvm.org/D148176