Outlives requirements are not implied in the return type of `async fn`, and generally for RPIT

Split from #102682

The following doesn't compile:

trait MyTrait<T> {
    async fn foo(&self) -> &T;
}

it gives the following:

error[E0311]: the parameter type `T` may not live long enough
 --> <source>:2:5
  |
2 |     async fn foo(&self) -> &T;
  |     ^^^^^^^^^^^^^-^^^^^^^^^^^^
  |     |            |
  |     |            the parameter type `T` must be valid for the anonymous lifetime as defined here...
  |     ...so that the reference type `&T` does not outlive the data it points at
  |
help: consider adding an explicit lifetime bound
  |
2 |     async fn foo<'a>(&'a self) -> &'a T where T: 'a;
  |                 ++++  ++           ++   +++++++++++

of course, a normal function is fine. We just don't imply that T: '_ here because it lowers to impl Future<Output = &'_ T>

Start by reproducing the async trait method example from the issue and comparing it with the equivalent normal function, which compiles. Investigate how async fn lowers to impl Future<Output = &'_ T> and how RPIT handles implied outlives requirements. Done means the example compiles without the explicit lifetime bound and has regression coverage.