Remove identity_future indirection

This was previously needed because the indirection used to hide some unexplained lifetime errors, which it turned out were related to the `min_choice` algorithm.

Removing the indirection also solves a couple of cycle errors, large moves and makes async blocks support the `#[track_caller]` annotation.

compiler/rustc_ast_lowering/src/item.rs

@@ -1180,7 +1180,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 },
             );
 
-            (this.arena.alloc_from_iter(parameters), this.expr(body.span, async_expr))
+            (this.arena.alloc_from_iter(parameters), async_expr)
         })
     }