c++: matching deduced template template parameters [PR67829]

During deduction, when the template of the argument for a bound ttp
is a template template parameter, we need to consider the
TEMPLATE_TEMPLATE_PARAMETER for matching rather than the TEMPLATE_DECL
thereof, because the canonical form of a template template parameter as
a template argument is the former tree, not the latter.

	PR c++/67829

gcc/cp/ChangeLog:

	* pt.c (unify) <case BOUND_TEMPLATE_TEMPLATE_PARM>: When
	the TEMPLATE_DECL of a BOUND_TEMPLATE_TEMPLATE_PARM argument is
	a template template parameter, adjust to the
	TEMPLATE_TEMPLATE_PARAMETER before falling through.

gcc/testsuite/ChangeLog:

	* g++.dg/template/ttp34.C: New test.
	* g++.dg/template/ttp34a.C: New test.
	* g++.dg/template/ttp34b.C: New test.