Rustdoc-Json: Report discriminant on all kinds of enum variant.

[aDotInTheVoid]

Rust now supports adding an explicit discriminant to all kinds of enum variant.

Eg:

#[repr(u8)]
enum Foo {
    Bar(i32) = 1,
    Baz { a: i32, b: i32 } = 2,
}

However we currently only report it for unit variants.

rust/src/rustdoc-json-types/lib.rs

Lines 339 to 370 in 7c99186

	pub enum Variant {
	/// A variant with no parentheses, and possible discriminant.
	///
	/// ```rust
	/// enum Demo {
	/// PlainVariant,
	/// PlainWithDiscriminant = 1,
	/// }
	/// ```
	Plain(Option<Discriminant>),
	/// A variant with unnamed fields.
	///
	/// Unlike most of json, `#[doc(hidden)]` fields will be given as `None`
	/// instead of being omitted, because order matters.
	///
	/// ```rust
	/// enum Demo {
	/// TupleVariant(i32),
	/// EmptyTupleVariant(),
	/// }
	/// ```
	Tuple(Vec<Option<Id>>),
	/// A variant with named fields.
	///
	/// ```rust
	/// enum Demo {
	/// StructVariant { x: i32 },
	/// EmptyStructVariant {},
	/// }
	/// ```
	Struct { fields: Vec<Id>, fields_stripped: bool },
	}

But it should be for all variants.

@rustbot claim