swift234

Introduce InFlightDiagnostic, a move-only class that will be used to emit "extra" information (ranges, Fix-Its, etc.) for a diagnostic. It's quite similar in design to Clang's DiagnosticBuilder, but slightly cleaner/more restrictive.

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