swift234

SILGen support for convenience factory and factory initializers.

These initializers are class methods, so provide them with the
metatype value rather than allocating an object. Moreover, they
generally return autoreleased, so we need to retain them out of the
autorelease pool before returning the produced 'self'.

Finishes the SILGen and IRGen portions of
<rdar://problem/16509024>. We still can't enable this in practice
because we're getting duplicate initializers, as predicated in the
hacky r16531.

Swift SVN r16555