c++ ICE with nested requirement as default tpl parm[PR94827]

Template headers are not incrementally updated as we parse its parameters.
We maintain a dummy level until the closing > when we replace the dummy with
a real parameter set.  requires processing was expecting a properly populated
arg_vec in current_template_parms, and then creates a self-mapping of parameters
from that.  But we don't need to do that, just teach map_arguments to look at
TREE_VALUE when args is NULL.

	* constraint.cc (map_arguments): If ARGS is null, it's a
	self-mapping of parms.
	(finish_nested_requirement): Do not pass argified
	current_template_parms to normalization.
	(tsubst_nested_requirement): Don't assert no template parms.