Do not consider synthesized RPITITs on missing items checks

[spastorino]

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 <https://github.com/rust-lang/rust/issues/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 <https://github.com/rust-lang/rust/issues/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

[compiler-errors]

@bors r+ rollup

[bors]

📌 Commit 364a5d4 has been approved by compiler-errors

It is now in the queue for this repository.