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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@154163 91177308-0d34-0410-b5e6-96231b3b80d8