[clang][HeaderSearch] Fix implicit module when using header maps

Previously, if a header was found via in a header map, and not just remapped.
we wouldn't also find the module it maps to when using implicit modules (for
module maps that were explicitly loaded).

This diff just updates these code paths to also locate the owning module via
`findUsableModuleForHeader`.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D103930