c++: Fix self-mapping in map_arguments [PR96531, PR97103]

c++: Fix self-mapping in map_arguments [PR96531, PR97103]

With r10-8077 we stopped passing the argified current_template_parms to
normalize_constraint_expression from finish_nested_requirement, and
instead made map_arguments perform a self-mapping of parameters when
args is NULL.  But we're currently not handling parameter packs and
BOUND_TEMPLATE_TEMPLATE_PARMs properly during this self-mapping, which
leads to ICEs later during satisfaction.

To properly handle self-mapping of a parameter pack, this patch
extends template_parm_to_arg to handle TEMPLATE_PARM_P nodes, and
makes map_arguments use it.  (This change revealed that the call to
template_parm_to_arg in convert_generic_types_to_packs was a no-op
because the argument 't' is never a TREE_LIST, so this patch
additionally removes this call.)

As for bound ttps, map_arguments before r10-8077 would map a
BOUND_TEMPLATE_TEMPLATE_PARM not to itself but to its underlying
TEMPLATE_TEMPLATE_PARM.  We could restore this behavior in
map_arguments, but since a bound ttp is not really a template parameter
it seems better to make keep_template_parm not give us a bound ttp in
the first place.  So this patch makes keep_template_parm return the
underlying ttp when it sees a bound ttp.

gcc/cp/ChangeLog:

	PR c++/96531
	PR c++/97103
	* constraint.cc (map_arguments): Call template_parm_to_arg
	in the self-mapping case.
	(finish_shorthand_constraint): No need to build a TREE_LIST
	before calling template_parm_to_arg.
	* pt.c (template_parm_to_arg): Rewrite to handle TEMPLATE_PARM_P
	nodes as well as DECL_TEMPLATE_PARM_P nodes, and to make the
	overlying TREE_LIST node optional.
	(keep_template_parm): Don't record a BOUND_TEMPLATE_TEMPLATE_PARM,
	instead record its corresponding TEMPLATE_TEMPLATE_PARM.
	(convert_generic_types_to_packs): Don't call
	template_parm_to_arg.

gcc/testsuite/ChangeLog:

	PR c++/96531
	PR c++/97103
	* g++.dg/cpp2a/concepts-ttp2.C: New test.
	* g++.dg/cpp2a/concepts-variadic1.C: New test.