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

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175272 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/SemaObjC/objc-literal-comparison.m b/test/SemaObjC/objc-literal-comparison.m
index 0a10582..95ebfb3 100644
--- a/test/SemaObjC/objc-literal-comparison.m
+++ b/test/SemaObjC/objc-literal-comparison.m
@@ -98,3 +98,6 @@
   RETURN_IF_NIL(@(1+1));
 }
 
+void PR15257(Class c) {
+  return c == @""; // expected-warning{{direct comparison of a string literal has undefined behavior}}
+}