[clang] Report the on-disk paths for inputs to module compiles

Since D135636, PCM files contain the "as requested" path of input files. The machinery for generating dependency files reports those paths as they appeared in the PCM file, which may confuse consumers that are not aware of VFS overlays that might've been in place at compile-time.

This patch makes sure the "use-external-name" setting is being respected when generating dependency files in modular builds by piping the paths serialized in PCMs through `FileEntryRef::getName()` before putting them into dependency files.

rdar://103459532

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D141644