swift234

When needed, emit a Objective-C .cxx_construct method to initialize stored properties.

When a class that will be allocated by the Objective-C runtime also
requires all of its stored properties have initial values, move that
complete initialization of the object into -.cxx_construct, which is
invoked by the runtime after +alloc.

Definite initialization doesn't like the resulting initializers (which
don't initialize any of their ivars) at all, so no end-to-end testing yet.

Swift SVN r12229