c++: Fix ICE with CTAD in concept [PR98611]

c++: Fix ICE with CTAD in concept [PR98611]

This patch teaches cp_walk_subtrees to visit the template represented
by a CTAD placeholder, which would otherwise be not visited during
find_template_parameters.  The template may be a template template
parameter (as in the first testcase), or it may implicitly use the
template parameters of an enclosing class template (as in the second
testcase), and in either case we need to visit this tree to record the
template parameters used therein for later satisfaction.

gcc/cp/ChangeLog:

	PR c++/98611
	* tree.c (cp_walk_subtrees) <case TEMPLATE_TYPE_PARM>: Visit
	the template of a CTAD placeholder.

gcc/testsuite/ChangeLog:

	PR c++/98611
	* g++.dg/cpp2a/concepts-ctad1.C: New test.
	* g++.dg/cpp2a/concepts-ctad2.C: New test.