Replace getAs with castAs to fix null dereference static analyzer warning.

Use castAs as we know the cast should succeed (and castAs will assert if it doesn't) and we're dereferencing it directly in the BuildRCBlockVarRecordLayout call.

Author: RKSimon

clang/lib/CodeGen/CGObjCMac.cpp

@@ -2558,9 +2558,8 @@ void CGObjCCommonMac::BuildRCRecordLayout(const llvm::StructLayout *RecLayout,
       }
       if (FQT->isRecordType() && ElCount) {
         int OldIndex = RunSkipBlockVars.size() - 1;
-        const RecordType *RT = FQT->getAs<RecordType>();
-        BuildRCBlockVarRecordLayout(RT, BytePos + FieldOffset,
-                                    HasUnion);
+        auto *RT = FQT->castAs<RecordType>();
+        BuildRCBlockVarRecordLayout(RT, BytePos + FieldOffset, HasUnion);
 
         // Replicate layout information for each array element. Note that
         // one element is already done.