swift234

Tighten up the semantics of unchecked downcasts (x as! T).

Explicit detect (and reject) conversions that aren't class
downcasts. We'll want to lift some of these restrictions later (see
<rdar://problem/14013456>), but for now we just reject them with a
decent diagnostic (rather than crashing).

When an explicit downcast is actually just a coercion, complain,
provide a Fix-It, and update the AST appropriately.

This also splits the checking of unchecked downcasts into two different
constraint systems: one for the context of expression, and one for the
subexpression, then compares the results. This eliminates the need to
model "can be downcast to" in the constraint solver, and makes it
easier to provide good diagnostics.



Swift SVN r5377