[CrossTU] Add a function to retrieve original source location.
A new function will be added to get the original SourceLocation
for a SourceLocation that was imported as result of getCrossTUDefinition.
The returned SourceLocation is in the context of the (original)
SourceManager for the original source file. Additionally the
ASTUnit object for that source file is returned. This is needed
to get a SourceManager to operate on with the returned source location.
The new function works if multiple different source files are loaded
with the same CrossTU context.
Reviewers: martong, shafik
Reviewed By: martong
Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, cfe-commits
Differential Revision: https://reviews.llvm.org/D65064