Fix an assertion hit when the serialized diagnostics writer receive a diagnostic

from the frontend when the location is invalid and the SourceManager null.

Instead of keeping the SourceManager object in DiagnosticRenderer, propagate it
to the calls accordingly (as reference when it is expected to not be null, or pointer
when it may be null).
This effectively makes DiagnosticRenderer not tied to a specific SourceManager,
removing a hack from TextDiagnosticPrinter.

rdar://11386874

llvm-svn: 156536