c++: P2308, Template parameter initialization (tests) [PR113800]

c++: P2308, Template parameter initialization (tests) [PR113800]

This proposal was implemented a long time ago by my r9-5271,
but it took me this long to verify that it still works as per P2308.

This patch adds assorted tests, both from clang and from [temp.arg.nontype].
Fortunately I did not discover any issues in the compiler.

	PR c++/113800
	DR 2450

gcc/testsuite/ChangeLog:

	* g++.dg/cpp26/pack-indexing15.C: New test.
	* g++.dg/cpp2a/nontype-class68.C: New test.
	* g++.dg/cpp2a/nontype-class69.C: New test.
	* g++.dg/cpp2a/nontype-class70.C: New test.
	* g++.dg/cpp2a/nontype-class71.C: New test.
	* g++.dg/cpp2a/nontype-class72.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>