Added support to C++ symbols rewriting.

This should simplify full C++ template instantiations.

Reviewed-by: Christian Kamm

src/libs/3rdparty/cplusplus/CPlusPlusForwardDeclarations.h

@@ -79,6 +79,9 @@ class ArrayType;
 class NamedType;
 
 // symbols
+class Clone;
+class Subst;
+
 class SymbolVisitor;
 class Symbol;
 class Scope;