c++: [[deprecated]] on template redecl [PR84542]

c++: [[deprecated]] on template redecl [PR84542]

The deprecated and unavailable attributes weren't working when used on
a template redeclaration ultimately because we weren't merging the
corresponding tree flags in duplicate_decls.

	PR c++/84542

gcc/cp/ChangeLog:

	* decl.cc (merge_attribute_bits): Merge TREE_DEPRECATED
	and TREE_UNAVAILABLE.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/attr-deprecated-2.C: No longer XFAIL.
	* g++.dg/ext/attr-unavailable-12.C: New test.