c++: passing partially inst ttp as ttp [PR110566]

c++: passing partially inst ttp as ttp [PR110566]

The previous fix doesn't work for partially instantiated ttps mainly
because most_general_template is a no-op for them.  This patch fixes
this by giving such ttps a DECL_TEMPLATE_INFO (extending the
r11-734-g2fb595f8348e16 fix) with which most_general_template can obtain
the original, unlowered ttp.

This patch additionally makes coerce_template_template_parms use the
correct amount of levels from the scope of a ttp argument.

	PR c++/110566
	PR c++/108179

gcc/cp/ChangeLog:

	* pt.cc (reduce_template_parm_level): Set DECL_TEMPLATE_INFO
	on the DECL_TEMPLATE_RESULT of the new ttp.
	(add_defaults_to_ttp): Make a copy of the original ttp's
	DECL_TEMPLATE_RESULT, and update this copy's DECL_TEMPLATE_INFO
	as well.
	(coerce_template_template_parms): Make sure 'scope_args' has
	the right amount of levels for the ttp argument.
	(most_general_template): Handle template template parameters.
	(rewrite_template_parm): Set DECL_TEMPLATE_RESULT on the
	DECL_TEMPLATE_RESULT of the new ttp.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/class-deduction115.C: New test.
	* g++.dg/template/ttp39.C: New test.