Auto merge of #88558 - fee1-dead:const-drop, r=oli-obk

Const drop

The changes are pretty primitive at this point. But at least it works. ^-^

Problems with the current change that I can think of now:
 - [x] `~const Drop` shouldn't change anything in the non-const world.
 - [x] types that do not have drop glues shouldn't fail to satisfy `~const Drop` in const contexts. `struct S { a: u8, b: u16 }` This might not fail for `needs_non_const_drop`, but it will fail in `rustc_trait_selection`.
 - [x] The current change accepts types that have `const Drop` impls but have non-const `Drop` glue.

Fixes #88424.

Significant Changes:

- `~const Drop` is no longer treated as a normal trait bound. In non-const contexts, this bound has no effect, but in const contexts, this restricts the input type and all of its transitive fields to either a) have a `const Drop` impl or b) can be trivially dropped (i.e. no drop glue)
- `T: ~const Drop` will not be linted like `T: Drop`.
- Instead of recursing and iterating through the type in `rustc_mir::transform::check_consts`, we use the trait system to special case `~const Drop`. See [`rustc_trait_selection::...::candidate_assembly#assemble_const_drop_candidates`](https://github.com/fee1-dead/rust/blob/const-drop/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs#L817) and others.

Changes not related to `const Drop`ping and/or changes that are insignificant:

 - `Node.constness_for_typeck` no longer returns `hir::Constness::Const` for type aliases in traits. This was previously used to hack how we determine default bound constness for items. But because we now use an explicit opt-in, it is no longer needed.
 - Removed `is_const_impl_raw` query. We have `impl_constness`, and the only existing use of that query uses `HirId`, which means we can just operate it with hir.
 - `ty::Destructor` now has a field `constness`, which represents the constness of the destructor.

r? `@oli-obk`

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1477,7 +1477,8 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
             }
             super::ImplSource::AutoImpl(..)
             | super::ImplSource::Builtin(..)
-            | super::ImplSource::TraitUpcasting(_) => {
+            | super::ImplSource::TraitUpcasting(_)
+            | super::ImplSource::ConstDrop(_) => {
                 // These traits have no associated types.
                 selcx.tcx().sess.delay_span_bug(
                     obligation.cause.span,
@@ -1549,7 +1550,8 @@ fn confirm_select_candidate<'cx, 'tcx>(
         | super::ImplSource::Param(..)
         | super::ImplSource::Builtin(..)
         | super::ImplSource::TraitUpcasting(_)
-        | super::ImplSource::TraitAlias(..) => {
+        | super::ImplSource::TraitAlias(..)
+        | super::ImplSource::ConstDrop(_) => {
             // we don't create Select candidates with this kind of resolution
             span_bug!(
                 obligation.cause.span,