attributes: silence clippy lints for #[instrument] on async functions for crates that warn on implicit_return (#3485)

The macro expands to a trialing if statement that is used as an implicit return value
for the function. That works, but for users who set `implicit_return = "deny"` then
this results in triggering clippy.

## Motivation

See https://github.com/rust-lang/rust-clippy/issues/16676

Minimum reproducible example:

**Cargo.toml:**

```
[package]
name = "clippy-implicit-return-instrument-repro"
edition = "2024"

[dependencies]
tracing = { version = "0.1.44", features = ["attributes"] }

[lints.clippy]
implicit_return = "deny"
```

**src/lib.rs:**

```rust
#[tracing::instrument]
pub async fn run() {
}
```

If you then run `cargo clippy` you get this result:

```
error: missing `return` statement
 --> src/lib.rs:1:1
  |
1 | #[tracing::instrument]
  | ^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.94.0/index.html#implicit_return
  = note: requested on the command line with `-D clippy::implicit-return`
  = note: this error originates in the attribute macro `tracing::instrument` (in Nightly builds, run with -Z macro-backtrace for more info)
```

## Solution

`#[allow(clippy::implicit_return)]` on the trailing return so it doesn't trigger clippy.

tracing-attributes/src/expand.rs

@@ -347,6 +347,7 @@ fn gen_block<B: ToTokens>(
         return quote!(
             let __tracing_attr_span = #span;
             let __tracing_instrument_future = #mk_fut;
+            #[allow(clippy::implicit_return)]
             if !__tracing_attr_span.is_disabled() {
                 #follows_from
                 ::tracing::Instrument::instrument(