Correct error recovery when missing 'class' in a template template parameter.

The diagnostic message correctly informs the user that they have omitted the
'class' keyword, but neither suggests this insertion as a fixit, nor attempts
to recover as if they had provided the keyword.

This fixes the recovery, adds the fixit, and adds a separate diagnostic and
corresponding replacement fixit for cases where the user wrote 'struct' or
'typename' instead of 'class' (suggested by Richard Smith as a possible common
mistake).

I'm not sure the diagnostic message for either the original or new cases feel
very Clang-esque, so I'm open to suggestions there. The fixit hints make it
fairly easy to see what's required, though.

llvm-svn: 153887