Stabilize type_alias_enum_variants in Rust 1.37.0

[Centril]

Stabilize #![feature(type_alias_enum_variants)] which allows type-relative resolution with highest priority to enum variants in both expression and pattern contexts. For example, you may now write:

enum Option<T> {
    None,
    Some(T),
}

type OptAlias<T> = Option<T>;

fn work_on_alias(x: Option<u8>) -> u8 {
    match x {
        OptAlias::Some(y) => y + 1,
        OptAlias::None => 0,
    }
}

Closes rust-lang/rfcs#2218
Closes #52118

r? @petrochenkov