This patch improves the C++ parser to accommodate the tools for
generating the functional specification from source code:
* Added support for class definitions prefixed with a
namespace as promoted by Genode's coding style.
* Improves robustness of the parsing of function arguments by
considering nameless arguments in function declarations, default
values, varargs.
* Consider const qualfiers in return types.
* Added support for the override, constexpr keywords.
* Parsing of overloaded operators.
* Improved handling of type definitions.
* Added parsing of template arguments.
* Handling of template constructors.
