swift234

Add Builtin.castReference and internal API _unsafeCastReference.

_unsafeCastReference allows casting of any references types, regardless
of whether they are references to objects or class existentials. The
implementation is responsible for converting between representations.

_unsafeCastReference provides a dynamic check to ensure that the source
and dest are both actually references. If not, the implementation will
trap at runtime. Generally, the optimizer can prove that the source
and dest are references, and promote this cast to an
unchecked_ref_cast bitcast. There is no dynamic check that the
references types are compatible.

This differs from unsafeDownCast in two ways:

(1) The source and dest types are not statically typed
AnyObjects. Therefore, unsafeCastReference can be used when the
surrounding code dynamically handles both reference and nonreference
types.

(2) The source and dest also need not dynamically conform to AnyObject.
Either side of the cast may be a class existential. The primary
requirement is that the source and dest refer to the same reference
counted object.

Swift SVN r32588