Default Associated Consts cannot use `Self::`

[strega-nil]

#![feature(associated_consts)]

trait Boop {
    const X: fn() = Self::x;
    fn x();
}

errors with

<anon>:4:21: 4:28 error: failed to resolve. Use of undeclared type or module `Self` [E0433]
<anon>:4     const X: fn() = Self::x;
                             ^~~~~~~
<anon>:4:21: 4:28 help: see the detailed explanation for E0433
<anon>:4:21: 4:28 error: unresolved name `Self::x` [E0425]
<anon>:4     const X: fn() = Self::x;
                             ^~~~~~~
<anon>:4:21: 4:28 help: see the detailed explanation for E0425

[petrochenkov]

Duplicate of #28809
(It's not only A-resolve, type checker support is also required.)