swift234

Rework how synthesis of initializers is performed.  Before, we would do it before
type checking any of the members of the type being analyzed, which leads to the
really exciting task of generating (e.g.) a memberwise initializer when you don't
know what the types of the members are.

Instead, synthesize these after the members are checked, which makes the code flow
much more like the other code synthesis stuff we do, and paves the way for future
improvements. NFC except one diagnostic in an error case (due to an order-of-checking 
change).

This resolves <rdar://problem/16921173> implicit constructor synthesization works without validating the members of the type


Swift SVN r25545