[clang][modules] Skip submodule & framework re-definitions in module maps

Before D150478, there were situations when Clang avoided parsing a module map because it was likely to re-define an already defined module (either by a PCM or by previously-found module map). Since Clang no longer performs that check and does parse the extra module map (due to the FW/FW_Private issue described in D150478), this patch re-implements the same semantics by skipping the duplicate definition of the framework module while parsing the module map.

Depends on D150478.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D150479