Rollup merge of #109414 - spastorino:new-rpitit-16, r=compiler-errors

Do not consider synthesized RPITITs on missing items checks

Without this patch for `tests/ui/impl-trait/in-trait/dont-project-to-rpitit-with-no-value.rs` we get ...

```
warning: the feature `return_position_impl_trait_in_trait` is incomplete and may not be safe to use and/or cause compiler crashes
 --> tests/ui/impl-trait/in-trait/dont-project-to-rpitit-with-no-value.rs:4:12
  |
4 | #![feature(return_position_impl_trait_in_trait)]
  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #91611 <rust-lang/rust#91611> for more information
  = note: `#[warn(incomplete_features)]` on by default

error[E0046]: not all trait items implemented, missing: `foo`, ``
  --> tests/ui/impl-trait/in-trait/dont-project-to-rpitit-with-no-value.rs:12:1
   |
8  |     fn foo(&self) -> impl Sized;
   |     ----------------------------
   |     |                |
   |     |                `` from trait
   |     `foo` from trait
...
12 | impl MyTrait for i32 {
   | ^^^^^^^^^^^^^^^^^^^^ missing `foo`, `` in implementation

error: aborting due to previous error; 1 warning emitted

For more information about this error, try `rustc --explain E0046`.
```

instead of ...

```
warning: the feature `return_position_impl_trait_in_trait` is incomplete and may not be safe to use and/or cause compiler crashes
  --> $DIR/dont-project-to-rpitit-with-no-value.rs:4:12
   |
LL | #![feature(return_position_impl_trait_in_trait)]
   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: see issue #91611 <rust-lang/rust#91611> for more information
   = note: `#[warn(incomplete_features)]` on by default

error[E0046]: not all trait items implemented, missing: `foo`
  --> $DIR/dont-project-to-rpitit-with-no-value.rs:12:1
   |
LL |     fn foo(&self) -> impl Sized;
   |     ---------------------------- `foo` from trait
...
LL | impl MyTrait for i32 {
   | ^^^^^^^^^^^^^^^^^^^^ missing `foo` in implementation

error: aborting due to previous error; 1 warning emitted

For more information about this error, try `rustc --explain E0046`.
```

r? `@compiler-errors`

Author: Dylan-DPC