Rollup merge of #127058 - compiler-errors:tighten-async-spans, r=oli-obk

Tighten `fn_decl_span` for async blocks

Tightens the span of `async {}` blocks in diagnostics, and subsequently async closures and async fns, by actually setting the `fn_decl_span` correctly. This is kinda a follow-up on #125078, but it fixes the problem in a more general way.

I think the diagnostics are significantly improved, since we no longer have a bunch of overlapping spans. I'll point out one caveat where I think the diagnostic may get a bit more confusing, but where I don't think it matters.

r? `@estebank` or `@oli-obk` or someone else on wg-diag or compiler i dont really care lol

Author: jhpratt

tests/ui/async-await/coroutine-desc.stderr

@@ -8,8 +8,8 @@ LL |     fun(async {}, async {});
    |     |   expected all arguments to be this `async` block type because they need to match the type of this parameter
    |     arguments to this function are incorrect
    |
-   = note: expected `async` block `{async block@$DIR/coroutine-desc.rs:10:9: 10:17}`
-              found `async` block `{async block@$DIR/coroutine-desc.rs:10:19: 10:27}`
+   = note: expected `async` block `{async block@$DIR/coroutine-desc.rs:10:9: 10:14}`
+              found `async` block `{async block@$DIR/coroutine-desc.rs:10:19: 10:24}`
    = note: no two async blocks, even if identical, have the same type
    = help: consider pinning your async block and casting it to a trait object
 note: function defined here

tests/ui/async-await/coroutine-not-future.stderr

@@ -26,11 +26,11 @@ note: required by a bound in `takes_coroutine`
 LL | fn takes_coroutine<ResumeTy>(_g: impl Coroutine<ResumeTy, Yield = (), Return = ()>) {}
    |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `takes_coroutine`
 
-error[E0277]: the trait bound `{async block@$DIR/coroutine-not-future.rs:39:21: 39:29}: Coroutine<_>` is not satisfied
+error[E0277]: the trait bound `{async block@$DIR/coroutine-not-future.rs:39:21: 39:26}: Coroutine<_>` is not satisfied
   --> $DIR/coroutine-not-future.rs:39:21
    |
 LL |     takes_coroutine(async {});
-   |     --------------- ^^^^^^^^ the trait `Coroutine<_>` is not implemented for `{async block@$DIR/coroutine-not-future.rs:39:21: 39:29}`
+   |     --------------- ^^^^^^^^ the trait `Coroutine<_>` is not implemented for `{async block@$DIR/coroutine-not-future.rs:39:21: 39:26}`
    |     |
    |     required by a bound introduced by this call
    |