Reland "[modules] Fix error about the same module being defined in different...

Reland "[modules] Fix error about the same module being defined in different .pcm files when using VFS overlays."

Fixing Windows buildbot by not using "BuildTemporaries/module.modulemap"
because it is interpreted as defining a module in "BuildTemporaries" directory.

Fix errors like
> module 'MultiPath' is defined in both 'path/to/modules.cache/3JR48BPRU7BCG/MultiPath-1352QHUF8RNMU.pcm' and 'path/to/modules.cache/3JR48BPRU7BCG/MultiPath-20HNSLLIUDDV1.pcm'

To avoid building extra identical modules `-ivfsoverlay` option is not a
part of the hash like "/3JR48BPRU7BCG/". And it is build system's
responsibility to provide `-ivfsoverlay` options that don't cause
observable differences.  We also need to make sure the hash like
"-1352QHUF8RNMU" is not affected by `-ivfsoverlay`. As this hash is
defined by the module map path, use the path prior to any VFS
remappings.

rdar://111921464

Differential Revision: https://reviews.llvm.org/D156749