internal: match_ast macro supports guard

[A4-Tacks]

Example

match_ast! {
    match parent {
        ast::Expr(it) if !it.is_block_like() => replace(it),
        _ => return,
    }
}

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[ChayimFriedman2]

This is so simple that I'm not opposed even though it's not used currently.

View changes since this review