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

compiler/rustc_ast/src/ast.rs

@@ -284,7 +284,7 @@ impl ParenthesizedArgs {
 
 pub use crate::node_id::{NodeId, CRATE_NODE_ID, DUMMY_NODE_ID};
 
-/// A modifier on a bound, e.g., `?Sized` or `?const Trait`.
+/// A modifier on a bound, e.g., `?Sized` or `~const Trait`.
 ///
 /// Negative bounds should also be handled here.
 #[derive(Copy, Clone, PartialEq, Eq, Encodable, Decodable, Debug)]
@@ -295,10 +295,10 @@ pub enum TraitBoundModifier {
     /// `?Trait`
     Maybe,
 
-    /// `?const Trait`
+    /// `~const Trait`
     MaybeConst,
 
-    /// `?const ?Trait`
+    /// `~const ?Trait`
     //
     // This parses but will be rejected during AST validation.
     MaybeConstMaybe,