Add support for parsing C++11 final classes

Such as:

  class X final {};

This no longer gives a syntax error.
This change has introduced one more shift-reduce conflict in the parser.
with a conflict with a C style variable declaration with name final:

  class X final;

resulting in a syntax error (for C++ not C). This is an an unusual style
for C++ code and more typical declarations do work:

  X final;

Closes #672

Examples/test-suite/cpp11_final_override.i

@@ -27,7 +27,7 @@ struct Base {
   virtual ~Base() {}
 };
 
-struct Derived /*final*/ : Base {
+struct Derived final : Base {
   virtual void stuff() const noexcept override final {}
   virtual void override1() const noexcept override;
   virtual void override2() const noexcept override;