SourceManager is passed by reference, allowing the SourceManager to be
associated with a specific translation unit, and not the entire execution
of the driver.

Modified all users of Diagnostics to comply with this new interface.

Integrated SourceManager as a member variable of TargetInfo. TargetInfo will
eventually be associated with a single translation unit (just like
SourceManager).

Made the SourceManager reference in ASTContext private. Provided accessor
getSourceManager() for clients to use instead. Modified clients to comply with
new interface.

llvm-svn: 44878