Decide on behavior of `anonymous_lifetime_in_impl_trait`

[traviscross]

The author has seemingly lost interest in this stabilization:

• Stabilize anonymous_lifetime_in_impl_trait #107378

However, we'd still be well-advised to finish our discussion about that so as to unblock anyone else from pursuing this stabilization.

We'll nominate this issue in its place.

cc @rust-lang/lang @rust-lang/types

@rustbot labels +C-discussion +I-lang-nominated +T-lang