never patterns: ! argument not detected as diverging on async fn

With #120104, a function with a ! pattern in a binding is detected as diverging:

fn never_arg(!: Void) -> ! {}
fn never_let(x: Void) -> ! {
  let ! = x;
}

This doesn't work the arguments of an async fn however:

async fn never_let(x: Void) -> ! {
  let ! = x;
} // ok
async fn never_arg(!: Void) -> ! {} // ERROR mistmatched types

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