CPlusPlus: Add toLink method to the Symbol

Basically move it from CppTools to CPlusPlus
to be able to use it there.

Change-Id: I0af80f93bdc029824397ceafdf940cb86c4382b0
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/cpptools/cppelementevaluator.cpp

@@ -129,7 +129,7 @@ CppDeclarableElement::CppDeclarableElement(Symbol *declaration)
     }
 
     tooltip = overview.prettyType(declaration->type(), qualifiedName);
-    link = CppTools::linkToSymbol(declaration);
+    link = declaration->toLink();
     helpMark = name;
 }