Fix async_yields_async wrongly unmangled macros

[profetia]

Closes #15552

changelog: [async_yields_async] fix wrong unmangle of macros

[rustbot]

r? @Jarcho

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[samueltardieu]

This is not directly linked to macros, but what if the return_expr requires parenthesis around it before applying .await?

                            format!("{}.await", snippet(cx, return_expr_span, "..")),

Shouldn't this use Sugg and Sugg::maybe_paren() here?

For example:

impl std::ops::Add for CustomFutureType {
    type Output = Self;

    fn add(self, other: Self) -> Self {
        Self
    }
}
    let _ = async || {
        CustomFutureType + CustomFutureType
    };

will not be processed properly.

If you don't mind, could you also fix this?

r? samueltardieu

[profetia]

@samueltardieu Done. Thank you!

[samueltardieu]

Can you squash the changes (and fix the CI of course)? Then it's good for me.

[profetia]

Done. Thank you!

[samueltardieu]

Some small changes left.