swift234

Lock down on function conversions that change the ABI of the function.

These haven't ever been safe in Swift's development because they require
generating thunks, and we currently don't do that. However, we were letting
existential conversions slip through the cracks because we consider them
subtypes, so that /metatype/ conversions work correctly. To be concrete:

  "let _: Any.Type = Int.self" is okay.
  "let _: (Int) -> Void = { (_: Any) -> Void in return }" is not.

We should implement this some day; that's rdar://problem/19517003.

This produces some lousy error messages, which I intend to fix soon.

Part of rdar://problem/19600325

Swift SVN r24915