swift234

[ClangImporter] Handle CF_RETURNS_[NOT_]RETAINED on parameters.

By declaring the parameter's retain count convention, we can avoid the
use of Unmanaged. (See test cases for more examples.)

The last piece of this puzzle is offering a sugared overload with multiple
return values (rdar://problem/20436785) but that will have to wait.

This requires changes to Clang; please update.

rdar://problem/20436757

Swift SVN r28216