swift234

Introduce the SIL instruction alloc_ref_dynamic.

alloc_ref_dynamic allocates an instance of a class type based on the
value in its metatype operand. Start emitting these instructions for
the allocating constructor of a complete object initializer (not yet
tested) and for the allocating constructor synthesized for an imported
Objective-C init method.

Still missing:
  - IRGen still does the same thing as alloc_ref right now. That
  change will follow.
  - There are devirtualization opportunities when we know the value of
  the metatype that would turn an alloc_ref_dynamic into an alloc_ref;
  I'm not planning to do this optimization.



Swift SVN r14560