swift234

Implement SILGen support for enum patterns, which are the most common and important
kind of pattern for let/else.  While I haven't done extensive testing, I believe
that let/else is wired up and working now.

Please feel free to kick the tires on let/else and file radars if you run into problems.

The only piece that I know is missing is support for is/as patterns (which will fail on
an llvm_unreachable).


Swift SVN r27135