`do catch` is accidentally stable in 2018 edition

[scottmcm]

This works with no #![feature]:

fn main() {
    let x: Option<_> = do catch { 4 };
}

https://play.rust-lang.org/?gist=ed21b1848a1114ed7f7fefc13bdb8880&version=nightly&mode=debug&edition=2018

I expect this to be fixed with #52602, but wanted to file explicitly for tracking.