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.