Overhaul Const.

Specifically, rename the `Const` struct as `ConstS` and re-introduce `Const` as
this:
```
pub struct Const<'tcx>(&'tcx Interned<ConstS>);
```
This now matches `Ty` and `Predicate` more closely, including using
pointer-based `eq` and `hash`.

Notable changes:
- `mk_const` now takes a `ConstS`.
- `Const` was copy, despite being 48 bytes. Now `ConstS` is not, so need a
  we need separate arena for it, because we can't use the `Dropless` one any
  more.
- Many `&'tcx Const<'tcx>`/`&Const<'tcx>` to `Const<'tcx>` changes
- Many `ct.ty` to `ct.ty()` and `ct.val` to `ct.val()` changes.
- Lots of tedious sigil fiddling.

compiler/rustc_mir_transform/src/const_prop.rs

@@ -484,7 +484,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
                 let err = ConstEvalErr::new(&self.ecx, error, Some(c.span));
                 if let Some(lint_root) = self.lint_root(source_info) {
                     let lint_only = match c.literal {
-                        ConstantKind::Ty(ct) => match ct.val {
+                        ConstantKind::Ty(ct) => match ct.val() {
                             // Promoteds must lint and not error as the user didn't ask for them
                             ConstKind::Unevaluated(ty::Unevaluated {
                                 def: _,
@@ -801,7 +801,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
     ) {
         if let Rvalue::Use(Operand::Constant(c)) = rval {
             match c.literal {
-                ConstantKind::Ty(c) if matches!(c.val, ConstKind::Unevaluated(..)) => {}
+                ConstantKind::Ty(c) if matches!(c.val(), ConstKind::Unevaluated(..)) => {}
                 _ => {
                     trace!("skipping replace of Rvalue::Use({:?} because it is already a const", c);
                     return;
@@ -875,7 +875,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
                                     literal: self
                                         .ecx
                                         .tcx
-                                        .mk_const(ty::Const {
+                                        .mk_const(ty::ConstS {
                                             ty,
                                             val: ty::ConstKind::Value(ConstValue::ByRef {
                                                 alloc,

compiler/rustc_mir_transform/src/inline.rs

@@ -625,7 +625,7 @@ impl<'tcx> Inliner<'tcx> {
                 caller_body.required_consts.extend(
                     callee_body.required_consts.iter().copied().filter(|&ct| {
                         match ct.literal.const_for_ty() {
-                            Some(ct) => matches!(ct.val, ConstKind::Unevaluated(_)),
+                            Some(ct) => matches!(ct.val(), ConstKind::Unevaluated(_)),
                             None => true,
                         }
                     }),

compiler/rustc_mir_transform/src/required_consts.rs

@@ -15,7 +15,7 @@ impl<'a, 'tcx> RequiredConstsVisitor<'a, 'tcx> {
 impl<'tcx> Visitor<'tcx> for RequiredConstsVisitor<'_, 'tcx> {
     fn visit_constant(&mut self, constant: &Constant<'tcx>, _: Location) {
         if let Some(ct) = constant.literal.const_for_ty() {
-            if let ConstKind::Unevaluated(_) = ct.val {
+            if let ConstKind::Unevaluated(_) = ct.val() {
                 self.required_consts.push(*constant);
             }
         }