Sema: Unnest early exit and remove an unnecessary bad cast.

cast<ObjCObjectPointerType> doesn't look through sugar, getAs does.
Fixes PR15257.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@175272 91177308-0d34-0410-b5e6-96231b3b80d8