[Sema][ObjC] Clean up possible null dereference.

It appears that the code is actually dead since unbridged-cast
placeholder types are created by calling CastOperation::complete and
ImplicitCastExprs are never passed to it.

Spotted by Vedant Kumar.

rdar://problem/31542226

llvm-svn: 302503

clang/lib/Sema/SemaExprObjC.cpp

@@ -4241,8 +4241,7 @@ void Sema::diagnoseARCUnbridgedCast(Expr *e) {
     castType = cast->getTypeAsWritten();
     CCK = CCK_OtherCast;
   } else {
-    castType = cast->getType();
-    CCK = CCK_ImplicitConversion;
+    llvm_unreachable("Unexpected ImplicitCastExpr");
   }
 
   ARCConversionTypeClass castACTC =