Fix parsing when type_specifier comes before type_qualifier.

If type_specifier comes after type_qualifier, then GISourceType representing type_specifier will be merely updated with qualifier flags. On the other hand, when this order is reversed the type qualifier used to be attached as a separate node through base_type (with CTYPE_INVALID), and interpreted incorrectly in transformer code.

This commit changes this behaviour so that information about type qualifiers is stored directly in GISourceType corresponding to type specifier. It also fixes analogous issue with storage_class_specifier and function_specifier.

From higher level viewpoint, it for example represents const char* and char const* in equivalent manner after parsing, and addresses issue #79 (closed).