For the C++17 changes to handling of template template parameter
matching, I replaced a lot of the old code.  But it seems that this
piece is still necessary when we aren't in C++17 mode, to handle the
case where we are comparing C<V> to set<int,void>, with different
numbers of arguments.  This is handled in C++17 mode by
coerce_ttp_args_for_tta, but that has other effects that we want to
limit to C++17 mode, at least for now.

Tested x86_64-pc-linux-gnu, applying to trunk and 7.