Revert "[VFS] RedirectingFileSystem only replace path if not already mapped"

This reverts commit 3fda0edc, which
breaks crash reproducers in very specific circumstances. Specifically,
since crash reproducers have `UseExternalNames` set to false, the
`File->getFileEntry().getDir()->getName()` call in `DoFrameworkLookup`
would use the *cached* directory name instead of the directory of the
looked-up file.

The plan is to re-commit this patch but to *add*
`ExposesExternalVFSPath` rather than replace `IsVFSMapped`.

Differential Revision: https://reviews.llvm.org/D123103