Split out a separate feature gate for impl trait in associated types

[oli-obk]

in #107645 it was decided that we'll take a new route for type alias impl trait. The exact route isn't clear yet, so while I'm working on implementing some of these proposed changes (e.g. in #110010) to be able to experiment with them, I will also work on stabilizing another sugar version first: impl trait in associated types. Similarly I'll look into creating feature gates for impl trait in const/static types.

This PR does nothing but split the feature gate, so that you need to enable a different feature gate for

impl Trait for Type {
    type Assoc = impl SomeTrait;
}

than what you need for type Foo = impl SomeTrait;

[rustbot]

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[jackh726]

r=me with CI green

[oli-obk]

@bors r=jackh726

[bors]

📌 Commit f263f88 has been approved by jackh726

It is now in the queue for this repository.