c++: constraint rewriting during ttp coercion [PR111485]

c++: constraint rewriting during ttp coercion [PR111485]

In order to compare the constraints of a ttp with that of its argument,
we rewrite the ttp's constraints in terms of the argument template's
template parameters.  The substitution to achieve this currently uses a
single level of template arguments, but that never does the right thing
because a ttp's template parameters always have level >= 2.  This patch
fixes this by including the outer template arguments in the substitution,
which ought to match the depth of the ttp.

The second testcase demonstrates it's better to substitute the concrete
outer template arguments instead of generic ones since a ttp's constraints
could depend on outer parameters.

	PR c++/111485

gcc/cp/ChangeLog:

	* pt.cc (is_compatible_template_arg): New parameter 'args'.
	Add the outer template arguments 'args' to 'new_args'.
	(convert_template_argument): Pass 'args' to
	is_compatible_template_arg.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-ttp5.C: New test.
	* g++.dg/cpp2a/concepts-ttp6.C: New test.

(cherry picked from commit 6f902a42b0afe3f3145bcb864695fc290b5acc3e)