c++: variadic ttp constraint subsumption [PR99904]

c++: variadic ttp constraint subsumption [PR99904]

Here we're crashing when level-lowering the variadic constraint C<Ts...>
on the template template parameter TT because tsubst_pack_expansion expects
processing_template_decl to be set during a partial substitution.

	PR c++/99904

gcc/cp/ChangeLog:

	* pt.c (is_compatible_template_arg): Set processing_template_decl
	around tsubst_constraint_info.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-ttp4.C: New test.

(cherry picked from commit 2e6e0d86a06389056d0e7fecc99c547420ad787a)