Begin work on supporting "N3206: Override control: Eliminating Attributes", from

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3206.htm

This lands support for parsing virt-specifier-seq after member functions, including the
contextual keywords 'final', and 'override'. The keywords are not yet used for anything.

llvm-svn: 123606