Introduce default_field_values feature

Initial implementation of `#[feature(default_field_values]`, proposed in https://github.com/rust-lang/rfcs/pull/3681.

Support default fields in enum struct variant

Allow default values in an enum struct variant definition:

```rust
pub enum Bar {
    Foo {
        bar: S = S,
        baz: i32 = 42 + 3,
    }
}
```

Allow using `..` without a base on an enum struct variant

```rust
Bar::Foo { .. }
```

`#[derive(Default)]` doesn't account for these as it is still gating `#[default]` only being allowed on unit variants.

Support `#[derive(Default)]` on enum struct variants with all defaulted fields

```rust
pub enum Bar {
    #[default]
    Foo {
        bar: S = S,
        baz: i32 = 42 + 3,
    }
}
```

Check for missing fields in typeck instead of mir_build.

Expand test with `const` param case (needs `generic_const_exprs` enabled).

Properly instantiate MIR const

The following works:

```rust
struct S<A> {
    a: Vec<A> = Vec::new(),
}
S::<i32> { .. }
```

Add lint for default fields that will always fail const-eval

We *allow* this to happen for API writers that might want to rely on users'
getting a compile error when using the default field, different to the error
that they would get when the field isn't default. We could change this to
*always* error instead of being a lint, if we wanted.

This will *not* catch errors for partially evaluated consts, like when the
expression relies on a const parameter.

Suggestions when encountering `Foo { .. }` without `#[feature(default_field_values)]`:

 - Suggest adding a base expression if there are missing fields.
 - Suggest enabling the feature if all the missing fields have optional values.
 - Suggest removing `..` if there are no missing fields.

compiler/rustc_hir_analysis/src/collect.rs

@@ -1021,12 +1021,12 @@ impl<'tcx> FieldUniquenessCheckContext<'tcx> {
     }
 }
 
-fn lower_variant(
-    tcx: TyCtxt<'_>,
+fn lower_variant<'tcx>(
+    tcx: TyCtxt<'tcx>,
     variant_did: Option<LocalDefId>,
     ident: Ident,
     discr: ty::VariantDiscr,
-    def: &hir::VariantData<'_>,
+    def: &hir::VariantData<'tcx>,
     adt_kind: ty::AdtKind,
     parent_did: LocalDefId,
 ) -> ty::VariantDef {
@@ -1042,6 +1042,7 @@ fn lower_variant(
             name: f.ident.name,
             vis: tcx.visibility(f.def_id),
             safety: f.safety,
+            value: f.default.map(|v| v.def_id.to_def_id()),
         })
         .collect();
     let recovered = match def {

compiler/rustc_hir_analysis/src/collect/type_of.rs

@@ -125,6 +125,12 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
             return ty;
         }
 
+        Node::Field(&hir::FieldDef { default: Some(c), def_id: field_def_id, .. })
+            if c.hir_id == hir_id =>
+        {
+            tcx.type_of(field_def_id).instantiate_identity()
+        }
+
         _ => Ty::new_error_with_message(
             tcx,
             span,