Improve move error diagnostic for AsyncFn closures

[heathdutton]

When an async closure captures a variable by move but is constrained to AsyncFn or AsyncFnMut, the error message now explains that the closure kind is the issue and points to the trait bound, similar to the existing diagnostic for Fn/FnMut closures.

Before:

error[E0507]: cannot move out of `foos` which is behind a shared reference
  --> src/lib.rs:12:20
   |
11 | async fn foo(foos: &mut [&mut Foo]) -> Result<(), ()> {
   |              ---- move occurs because `foos` has type...
12 |     in_transaction(async || -> Result<(), ()> {
   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^ `foos` is moved here
13 |         for foo in foos {
   |                    ---- variable moved due to use in coroutine

After:

error[E0507]: cannot move out of `y`, a captured variable in an `AsyncFn` closure
  --> src/lib.rs:9:10
   |
LL |     let y = vec![format!("World")];
   |         - captured outer variable
LL |     call(async || {
   |          ^^^^^^^^ captured by this `AsyncFn` closure
...
help: `AsyncFn` and `AsyncFnMut` closures require captured values to be able
      to be consumed multiple times, but `AsyncFnOnce` closures may consume
      them only once
  --> src/lib.rs:5:27
   |
LL | fn call<F>(_: F) where F: AsyncFn() {}
   |                           ^^^^^^^^^

Fixes #150174

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