Fix segmentation fault when mixing -Rpass with #line.

Summary:
When using #line directives, FileManager::getFile() will return a nil
entry. This triggers an assert in translateFileLineCol().

This patch handles nil FileEntry instances by emitting a note that the
location could not be translated back to a SourceLocation. I don't
really like this solution, but we are translating presumed locations,
so some information has already been lost.

Reviewers: rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D3625