swift234

Add Builtin.unreachable().

This will allow stdlib code to explicitly mark branches it knows to be unreachable. Make this work with SIL diagnostics by lowering the builtin to a normal builtin_function_ref/apply in SILGen, and special-case handling the builtin in DCE by removing the apply along with the following dead instructions when we recognize an unreachable block.

Swift SVN r16745