clang-tidy/cppcoreguidelines/ProTypeMemberInitCheck.cpp: Use raw_string_ostream::str() to flush the buffer explicitly.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@267290 91177308-0d34-0410-b5e6-96231b3b80d8

clang-tidy/cppcoreguidelines/ProTypeMemberInitCheck.cpp

@@ -145,7 +145,7 @@ struct IntializerInsertion {
       Stream << ", " << joined << "()";
       break;
     }
-    return Code;
+    return Stream.str();
   }
 
   InitializerPlacement Placement;