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

clippy_lints/src/suspicious_operation_groupings.rs

@@ -549,7 +549,7 @@ fn ident_difference_expr_with_base_location(
         | (Assign(_, _, _), Assign(_, _, _))
         | (TryBlock(_), TryBlock(_))
         | (Await(_, _), Await(_, _))
-        | (Gen(_, _, _), Gen(_, _, _))
+        | (Gen(_, _, _, _), Gen(_, _, _, _))
         | (Block(_, _), Block(_, _))
         | (Closure(_), Closure(_))
         | (Match(_, _, _), Match(_, _, _))