catch blocks can produce non-Try values

[scottmcm]

This works today, and I think it's not supposed to:

let x: i32 = do catch { 4 };

Repro: https://play.rust-lang.org/?gist=0c5b165abf4c06ff5821550986db4531&version=nightly

The value that comes out of a catch seems like it's supposed to only be something that's Try, since otherwise you cannot use ? inside the catch. (Magic ok-wrapping could make the block itself legal, but it'd still need to return some impl Try<Ok=i32>, not a raw i32.)

cc catch tracking issue: #31436

[kennytm]

cc #41414.