attributes: allow clippy::unreachable warning (#2356)

## Motivation

PR #2270 added an unreachable branch with an explicit return value to
`#[instrument]` in `async fn`s in order to fix type inference issues.
That PR added the appropriate `#[allow]` attribute for the Rust
compiler's unreachable code linting, but not Clippy's, so a Clippy
warning is still emitted. See:
#2270 (comment)

## Solution

Adding the clippy lint warning as discussed here:
#2270 (comment)

tracing-attributes/src/expand.rs

@@ -64,7 +64,7 @@ pub(crate) fn gen_function<'a, B: ToTokens + 'a>(
     // unreachable, but does affect inference, so it needs to be written
     // exactly that way for it to do its magic.
     let fake_return_edge = quote_spanned! {return_span=>
-        #[allow(unreachable_code, clippy::diverging_sub_expression, clippy::let_unit_value)]
+        #[allow(unreachable_code, clippy::diverging_sub_expression, clippy::let_unit_value, clippy::unreachable)]
         if false {
             let __tracing_attr_fake_return: #return_type =
                 unreachable!("this is just for type inference, and is unreachable code");