c++: vtable referring to "unavailable" virtual fn [PR116606]

c++: vtable referring to "unavailable" virtual fn [PR116606]

mark_vtable_entries already has

   /* It's OK for the vtable to refer to deprecated virtual functions.  */
   warning_sentinel w(warn_deprecated_decl);

but that doesn't cover __attribute__((unavailable)).  We can use the
following override to cover both.

	PR c++/116606

gcc/cp/ChangeLog:

	* decl2.cc (mark_vtable_entries): Temporarily override deprecated_state to
	UNAVAILABLE_DEPRECATED_SUPPRESS.  Remove a warning_sentinel.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/attr-unavailable-13.C: New test.