* There was an edge case where the arena could be destroyed twice on
error: once from the arena itself and once from the decl destruction.
* The type of the created decl was incorrect (it should have been the
pointer child type), but it's not required anyway, so it's now just
initialized to anyopaque (which more accurately reflects what's
actually at that memory, since e.g. [*]T may correspond to nothing).
* A runtime bitcast of the pointer was performed, meaning @extern didn't
work at comptime. This is unnecessary: the decl_ref can just be
initialized with the correct pointer type.
