Fix grammar for LiteralPattern regarding -

[traviscross]

We had documented that only numeric literals in patterns can be prefixed by - (minus), but the Rust parser happily accepts a minus ahead of all literals in patterns. E.g.:

#[cfg(any())]
match () {
    -true | -false => (),
    -'x' => (),
    -b'x' => (),
    -"x" => (),
    -r"x" => (),
    -br"x" => (),
    -c"x" => (),
    -cr"x" => (),
    -1 => (),
    -1.1 => (),
}

In the compiler, this happens in Parser::parse_literal_maybe_minus and Token::can_begin_literal_maybe_minus.

Let's fix this by defining LiteralPattern as a LiteralExpression optionally prefixed by the minus sign. This better matches how the rustc AST models this.

cc @ehuss

[traviscross]

I reworded this a bit, as it seemed to be saying two conflicting things. On the one hand, "literal patterns" match exactly what's parsed as a literal. But on the other, it said that "literal patterns" accept a minus sign, which isn't part of the literal. It seems better to just say that literals in patterns may be prefixed by an optional minus sign.

[ehuss]

Thanks!