swift234

In "x as T", type-check the subexpression "x" using "T" as the context type.

Using "T" as the contextual type, either for an implicit conversion
(in the coercion case) or as a downcast (for the checked-cast case),
opens up more type-inference opportunities. Most importantly, it
allows coercions such as "1 as UInt32" (important for
<rdar://problem/15283100>). Additionally, it allows one to omit
generic arguments within the type we're casting to.

Some additional cleanup to follow.


Swift SVN r10799