c++: Fix up user_provided_p [PR81349]

c++: Fix up user_provided_p [PR81349]

The standard says: "A function is user-provided if it is user-declared and
not explicitly defaulted or deleted on its first declaration."
I don't see anything about function templates having different rules here,
but user_provided_p does return true for all TEMPLATE_DECLs.

The following patch fixes it by treating as user-provided only templates
that aren't deleted.

2020-03-26  Jakub Jelinek  <jakub@redhat.com>

	PR c++/81349
	* class.c (user_provided_p): Use STRIP_TEMPLATE instead of returning
	true for all TEMPLATE_DECLs.

	* g++.dg/cpp1z/pr81349.C: New test.