swift234

[ClangImporter] Allow performSelector... methods, but make the result Unmanaged.

Per discussion, there are certain times where our APIs really want you to use
the performSelector family of methods (e.g. when the framework hands you a selector
and expects you to call it upon completion).

Although the methods still aren't type-safe, we are at least making the result
Unmanaged so that you're forced to think about whether it's +1 or +0 before you
use it, and so that the compiler doesn't accidentally try to retain a non-object
pointer.

This commit also removes the blocks on the makeObjectsPerformSelector... methods,
but Foundation plans to add NS_SWIFT_UNAVAILABLE there (see rdar://problem/21150180).

rdar://problem/21150277

Swift SVN r30044