This was from master branch commit
c93e0d86187cb589d6726acd36f741f3d87a96be. Since standalone test are
completely reworked, I had to resolve the merge conflict later, in this
commit.
* 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.
