Parse anonymous struct field syntax

[dtolnay]

Part of RFC 2102.

• RFC: Unnamed fields of struct and union type rust-lang/rfcs#2102
• Tracking issue: Tracking issue for RFC 2102, "Unnamed fields of struct and union type" rust-lang/rust#49804
• Example usage: https://github.com/rust-lang/rust/blob/e3c71f1e33b026dea7c9ca7c1c4554e63f56a0da/src/test/pretty/anonymous-types.rs

struct Foo {
    _: union {
        _: struct {
            a: u8,
            b: u16,
        },
        c: u32,
    },
}