Introduce ~const

- [x] Removed `?const` and change uses of `?const`
 - [x] Added `~const` to the AST. It is gated behind const_trait_impl.
 - [x] Validate `~const` in ast_validation.
 - [ ] Add enum `BoundConstness` to the HIR. (With variants `NotConst` and
 `ConstIfConst` allowing future extensions)
 - [ ] Adjust trait selection and pre-existing code to use `BoundConstness`.
 - [ ] Optional steps (*for this PR, obviously*)
      - [ ] Fix #88155
      - [ ] Do something with constness bounds in chalk

src/librustdoc/html/format.rs

@@ -387,7 +387,7 @@ impl clean::GenericBound {
                 let modifier_str = match modifier {
                     hir::TraitBoundModifier::None => "",
                     hir::TraitBoundModifier::Maybe => "?",
-                    hir::TraitBoundModifier::MaybeConst => "?const",
+                    hir::TraitBoundModifier::MaybeConst => "~const",
                 };
                 if f.alternate() {
                     write!(f, "{}{:#}", modifier_str, ty.print(cx))