C++: Fix "is constant" check

The check was supposed to check for constant type, not constant value.
This fixes a false negative that appeared in
`LargeParameter/test.cpp:106`.

cpp/ql/src/semmle/code/cpp/dataflow/EscapesTree.qll

@@ -223,7 +223,7 @@ private predicate lvalueMayEscapeMutablyAt(Expr e) {
   lvalueMayEscapeAt(e) and
   // A qualifier of a call to a const member function is converted to a const
   // class type.
-  not e.isConstant()
+  not e.getType().isConst()
 }
 
 private predicate addressFromVariableAccess(VariableAccess va, Expr e) {