Improve diagnostics for mismatched type in async main

Previously, we wrapped the body of the `async main` function in an
`async` block, which we passed to `block_on`. However, `block_on` is
generic, so an incorrect return type ends up creating a diagnostic
pointing a `block_on`, not the user's code. Since the call to `block_on`
is generated by the `#[tokio::main]` macro, it ended up with a span of
the `#[tokio::main]` attribute, producing a confusing diagnostic.

We now wrap the body of the `async main` function in a new
`async main_inner` function. This asserts a return type of `()` earlier
on, producing a diagnostic.

Given this code:

```rust
#[tokio::main]
async fn main() {
    Ok(())
}
```

We currently produce the error:

```
error[E0308]: mismatched types
 --> src/main.rs:1:1
  |
1 | #[tokio::main]
  | ^^^^^^^^^^^^^^- help: try adding a semicolon: `;`
  | |
  | expected `()`, found enum `std::result::Result`
  |
  = note: expected unit type `()`
                  found enum `std::result::Result<(), _>`
  = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)
```

With this PR, we produce:

```
error[E0308]: mismatched types
 --> src/main.rs:3:5
  |
3 |     Ok(())
  |     ^^^^^^- help: try adding a semicolon: `;`
  |     |
  |     expected `()`, found enum `std::result::Result`
  |
  = note: expected unit type `()`
                  found enum `std::result::Result<(), _>`
```

tokio-macros/src/entry.rs

@@ -244,11 +244,15 @@ fn parse_knobs(
         #header
         #(#attrs)*
         #vis #sig {
+            // Assert a return type of `()`,
+            // which produces a better diagnostic
+            async fn main_inner() -> () #body
+
             #rt
                 .enable_all()
                 .build()
                 .unwrap()
-                .block_on(async { #body })
+                .block_on(main_inner())
         }
     };