Fix RPIT in default async trait method

[Swatinem]

Removing the identity_future from the async lowering in #104833 will regress the tests/ui/impl-trait/in-trait/default-body-with-rpit.rs test.

This is deemed acceptable for now as that feature is still unstable, as long as we track progress to fix it, hence this issue.

See also #104833 (comment), CC @compiler-errors