c++: Attribute deprecated/unavailable divergence

c++: Attribute deprecated/unavailable divergence

Attributes deprecated and unavailable are largely the same, except
that the former produces a warning whereas the latter produces an error.
So is_late_template_attribute should treat them the same.  Confirmed by
Iain that this divergence is not intentional:
<https://gcc.gnu.org/pipermail/gcc-patches/2022-February/591007.html>.

gcc/cp/ChangeLog:

	* decl2.cc (is_late_template_attribute): Do not defer attribute
	unavailable.
	* pt.cc (tsubst_enum): Set TREE_UNAVAILABLE.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/attr-unavailable-9.C: Add dg-error.