actually render emitted diagnostics. This is both a useful
generalization (we expect to have a number of other
DiagnosticConsumers down the road, as Clang does) and is also
important now to avoid a layering violation when adjusting the source
location at the end of a SourceRange to the end of the token.
Swift SVN r850
Note that I had to re-implement llvm::Optional, since it lamely requires a default constructor. At some point, I'll push a proper implementation back to LLVM.
Swift SVN r845
