swift234

Convert unsafeDownCast to use Builtin.castReference.

Generally avoid casting to a raw pointer unless a raw pointer is needed.
Builtin.castReference is now the canonical way to cast reference types.
Canonical generally = better optimizer. In practice this will reduce to
the same SIL since the stdlib code will be optimized.

There are enough unit tests for unsafeDowncast and Builtin.castReference.

I measured no performance regressions.

Swift SVN r32646