[Bug c++/101519] Improve diagnostic for template name used as base class

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101519

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
See also PR 101526 which requests another improvement to the same diagnostic,
but with a different cause.