RPITIT with GAT fails to enforce `where Self: 'a` bound

[aliemjay]

The following compiles after the stabilization of RPITIT/AFIT in #115822:

trait Trait {
    type Gat<'a>;
    async fn foo(&self) -> Self::Gat<'_>;
}

but it should require adding where Self: 'a bound to Gat similar to the non-async version (see #87479):

trait Trait {
    type Gat<'a>;
    //~^ ERROR missing required bound on `Gat`
    //~| HELP add the required where clause: `where Self: 'a`

    fn foo(&self) -> Self::Gat<'_>;
}

cc @compiler-errors