[PCH] Allow VFS to be used for tests that generate PCH files

When using a virtual file-system (VFS) and a preamble file (PCH) is generated,
it is generated on-disk in the real file-system instead of in the VFS (which
makes sense, since the VFS is read-only). However, when subsequently reading
the generated PCH, the frontend passes through the VFS it has been given --
resulting in an error and a failed parse (since the VFS doesn't contain the
PCH; the real filesystem does).

This patch fixes that by detecting when a VFS is being used for a parse that
needs to work with a PCH file, and creating an overlay VFS that includes the
PCH file from the real file-system.

This allows tests to be written which make use of both PCH files and a VFS.

Differential Revision: https://reviews.llvm.org/D37474

llvm-svn: 312917