[clang][deps] Disable implicit module maps

Since D113473, we don't report any module map files via `-fmodule-map-file=` in explicit builds. The ultimate goal here is to make sure Clang doesn't open/read/parse/evaluate unnecessary module maps.

However, implicit module maps still end up reading all reachable module maps. This patch disables implicit module maps in explicit builds.

Unfortunately, we still need to report some module map files that aren't encoded in PCM files of dependencies: module maps that are necessary to correctly evaluate includes in modules marked as `[no_undeclared_includes]`.

Depends on D120464.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D120465