Rollup merge of #108551 - compiler-errors:rpitit-bad-spec, r=oli-obk

Descriptive error when users try to combine RPITIT/AFIT with specialization

Previously we failed with some esoteric error like:

```
error[E0053]: method `foo` has an incompatible type for trait
  --> $DIR/dont-project-to-specializable-projection.rs:14:35
   |
LL |     default async fn foo(_: T) -> &'static str {
   |                                   ^^^^^^^^^^^^ expected associated type, found future
   |
note: type in trait
  --> $DIR/dont-project-to-specializable-projection.rs:10:27
   |
LL |     async fn foo(_: T) -> &'static str;
   |                           ^^^^^^^^^^^^
   = note: expected signature `fn(_) -> impl Future<Output = &'static str>`
              found signature `fn(_) -> impl Future<Output = &'static str>`
```

Now we error like:

```
error: async associated function in trait cannot be specialized
  --> $DIR/dont-project-to-specializable-projection.rs:14:5
   |
LL |     default async fn foo(_: T) -> &'static str {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: specialization behaves in inconsistent and surprising ways with `#![feature(async_fn_in_trait)]`, and for now is disallowed
```

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