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_middle/src/ty/mod.rs

@@ -1364,6 +1364,7 @@ pub struct FieldDef {
     pub name: Symbol,
     pub vis: Visibility<DefId>,
     pub safety: hir::Safety,
+    pub value: Option<DefId>,
 }
 
 impl PartialEq for FieldDef {
@@ -1376,9 +1377,9 @@ impl PartialEq for FieldDef {
         // of `FieldDef` changes, a compile-error will be produced, reminding
         // us to revisit this assumption.
 
-        let Self { did: lhs_did, name: _, vis: _, safety: _ } = &self;
+        let Self { did: lhs_did, name: _, vis: _, safety: _, value: _ } = &self;
 
-        let Self { did: rhs_did, name: _, vis: _, safety: _ } = other;
+        let Self { did: rhs_did, name: _, vis: _, safety: _, value: _ } = other;
 
         let res = lhs_did == rhs_did;
 
@@ -1405,7 +1406,7 @@ impl Hash for FieldDef {
         // of `FieldDef` changes, a compile-error will be produced, reminding
         // us to revisit this assumption.
 
-        let Self { did, name: _, vis: _, safety: _ } = &self;
+        let Self { did, name: _, vis: _, safety: _, value: _ } = &self;
 
         did.hash(s)
     }