C++: Exclude copy assignment in LargeParameter.ql

The purpose of the copy assignment operator is to copy the object, so we
should not complain that a copy happens when passing the parameter. See
https://en.wikibooks.org/wiki/More_C++_Idioms/Copy-and-swap for details.

cpp/ql/test/query-tests/Critical/LargeParameter/LargeParameter.expected

@@ -1,4 +1,3 @@
 | test.cpp:16:13:16:14 | _t | This parameter of type $@ is 4096 bytes - consider passing a pointer/reference instead. | test.cpp:6:8:6:20 | myLargeStruct | myLargeStruct |
 | test.cpp:24:44:24:48 | mtc_t | This parameter of type $@ is 4096 bytes - consider passing a pointer/reference instead. | test.cpp:11:7:11:21 | myTemplateClass<myLargeStruct> | myTemplateClass<myLargeStruct> |
 | test.cpp:28:49:28:49 | b | This parameter of type $@ is 4096 bytes - consider passing a pointer/reference instead. | test.cpp:6:8:6:20 | myLargeStruct | myLargeStruct |
-| test.cpp:52:52:52:54 | rhs | This parameter of type $@ is 4096 bytes - consider passing a pointer/reference instead. | test.cpp:48:8:48:25 | CustomAssignmentOp | CustomAssignmentOp |

cpp/ql/test/query-tests/Critical/LargeParameter/test.cpp

@@ -49,6 +49,6 @@ struct CustomAssignmentOp {
   // GOOD: it's an accepted pattern to implement copy assignment via copy and
   // swap. This delegates the resource management involved in copying to the
   // copy constructor so that logic only has to be written once.
-  CustomAssignmentOp &operator=(CustomAssignmentOp rhs); // FALSE POSITIVE
+  CustomAssignmentOp &operator=(CustomAssignmentOp rhs);
   char data[4096];
 };