Add convert unwrap_or_else for convert_bool_then

[A4-Tacks]

Example

//- minicore:bool_impl
fn main() {
    true.t$0hen(|| 3).unwrap_or_else(|| 4)
}

Old output:

//- minicore:bool_impl
fn main() {
    if true {
        Some(3)
    } else {
        None
    }.unwrap_or_else(|| 4)
}

This PR current output:

//- minicore:bool_impl
fn main() {
    if true {
        3
    } else {
        4
    }
}

[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.