fix cycle error when a static and a promoted are mutually recursive

This also now allows promoteds everywhere to point to 'extern static', because why not?
We still check that constants cannot transitively reach 'extern static' through references.
(We allow it through raw pointers.)

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -356,22 +356,13 @@ pub fn const_validate_mplace<'mir, 'tcx>(
     let mut inner = false;
     while let Some((mplace, path)) = ref_tracking.todo.pop() {
         let mode = match ecx.tcx.static_mutability(cid.instance.def_id()) {
-            Some(_) if cid.promoted.is_some() => {
-                // Promoteds in statics are consts that re allowed to point to statics.
-                CtfeValidationMode::Const {
-                    allow_immutable_unsafe_cell: false,
-                    allow_extern_static_ptrs: true,
-                }
-            }
+            _ if cid.promoted.is_some() => CtfeValidationMode::Promoted,
             Some(mutbl) => CtfeValidationMode::Static { mutbl }, // a `static`
             None => {
                 // In normal `const` (not promoted), the outermost allocation is always only copied,
                 // so having `UnsafeCell` in there is okay despite them being in immutable memory.
                 let allow_immutable_unsafe_cell = cid.promoted.is_none() && !inner;
-                CtfeValidationMode::Const {
-                    allow_immutable_unsafe_cell,
-                    allow_extern_static_ptrs: false,
-                }
+                CtfeValidationMode::Const { allow_immutable_unsafe_cell }
             }
         };
         ecx.const_validate_operand(&mplace.into(), path, &mut ref_tracking, mode)?;