The implied deref to statics introduced by HIR->THIR lowering is only
used to create place expressions, it lacks unsafe semantics.
It is also confusing, as there is no visible `*ident` in the source.
For both classes of "unsafe static" (extern static and static mut)
allow this operation.
We lack a clear story around `thread_local! { static mut }`, which
is actually its own category of item that reuses the static syntax but
has its own rules. It's possible they should be similarly included, but
in the absence of a good reason one way or another, we do not bless it.
