[ASTImporter] Improve import of FileID.
Even if the content cache has a directory and filename, it may be a virtual file.
The old code returned with error in this case, but it is worth to try to handle
the file as it were a memory buffer.
Reviewers: a.sidorin, shafik, martong, a_sidorin
Reviewed By: shafik
Subscribers: efriedma, rnkovacs, cfe-commits, dkrupp, martong, Szelethus, gamesh411
Differential Revision: https://reviews.llvm.org/D57590