c++: placeholder type constraint and argument pack [PR99815]

c++: placeholder type constraint and argument pack [PR99815]

When checking dependence of a placeholder type constraint, if the first
template argument of the constraint is an argument pack, we need to
expand it in order to properly separate the implicit 'auto' argument
from the rest.

gcc/cp/ChangeLog:

	PR c++/99815
	* pt.c (placeholder_type_constraint_dependent_p): Expand
	argument packs to separate the first non-pack argument
	from the rest.

gcc/testsuite/ChangeLog:

	PR c++/99815
	* g++.dg/cpp2a/concepts-placeholder5.C: New test.