c++: Add !TYPE_P assert to type_dependent_expression_p [PR99080]

As discussed here:
<https://gcc.gnu.org/pipermail/gcc-patches/2021-February/564629.html>,
type_dependent_expression_p should not be called with a type argument.

I promised I'd add an assert so here it is.  One place needed adjusting.

	PR c++/99080

gcc/cp/ChangeLog:

	* pt.cc (type_dependent_expression_p): Assert !TYPE_P.
	* semantics.cc (finish_id_expression_1): Handle UNBOUND_CLASS_TEMPLATE
	specifically.