Honour 'use-external-names' in FileManager

Pass through the externally-visible names that we got from the VFS down
to FileManager, and test that this is the name showing up in __FILE__,
diagnostics, and debug information.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202442 91177308-0d34-0410-b5e6-96231b3b80d8