Auto merge of #120712 - compiler-errors:async-closures-harmonize, r=oli-obk

Harmonize `AsyncFn` implementations, make async closures conditionally impl `Fn*` traits

This PR implements several changes to the built-in and libcore-provided implementations of `Fn*` and `AsyncFn*` to address two problems:
1. async closures do not implement the `Fn*` family traits, leading to breakage: https://crater-reports.s3.amazonaws.com/pr-120361/index.html
2. *references* to async closures do not implement `AsyncFn*`, as a consequence of the existing blanket impls of the shape `AsyncFn for F where F: Fn, F::Output: Future`.

In order to fix (1.), we implement `Fn` traits appropriately for async closures. It turns out that async closures can:
* always implement `FnOnce`, meaning that they're drop-in compatible with `FnOnce`-bound combinators like `Option::map`.
* conditionally implement `Fn`/`FnMut` if they have no captures, which means that existing usages of async closures should *probably* work without breakage (crater checking this: https://github.com/rust-lang/rust/pull/120712#issuecomment-1930587805).

In order to fix (2.), we make all of the built-in callables implement `AsyncFn*` via built-in impls, and instead adjust the blanket impls for `AsyncFn*` provided by libcore to match the blanket impls for `Fn*`.

compiler/rustc_infer/src/traits/error_reporting/mod.rs

@@ -177,12 +177,13 @@ pub fn report_object_safety_error<'tcx>(
             )));
         }
         impls => {
-            let types = impls
+            let mut types = impls
                 .iter()
                 .map(|t| {
                     with_no_trimmed_paths!(format!("  {}", tcx.type_of(*t).instantiate_identity(),))
                 })
                 .collect::<Vec<_>>();
+            types.sort();
             err.help(format!(
                 "the following types implement the trait, consider defining an enum where each \
                  variant holds one of these types, implementing `{}` for this new enum and using \