Restrict fixit for missing 'class' in template template parameters.

Based on Doug's feedback to r153887 this omits the FixIt if the following token
isn't syntactically valid for the context. (not a comma, '...', identifier,
'>', or '>>')

There's a bunch of work to handle the '>>' case, but it makes for a much more
pleasant diagnostic in this case.

llvm-svn: 154163