c++: alias CTAD and template template parm [PR114377]

c++: alias CTAD and template template parm [PR114377]

To match all the other places that pull a _TEMPLATE_PARM out of a
_DECL (get_template_parm_index, etc.).

This change is too small to be legally significant for copyright.

	PR c++/114377

gcc/cp/ChangeLog:

	* pt.cc (find_template_parameter_info::found): Use TREE_TYPE for
	TEMPLATE_DECL instead of DECL_INITIAL.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/class-deduction-alias19.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>
(cherry picked from commit 801e82acd6b4f0cf863529875947e394899ea7b9)