Wrong assoc. item completion in trait impl

[jonas-schievink]

trait Tr {
    type Assoc;
}

impl Tr for () {
    type$0
}

Applying the type Assoc = completion results in:

trait Tr {
    type Assoc;
}

impl Tr for () {
    typetype Assoc = $0;
}

This doesn't seem to happen on nightlies that are a few days old, so this is a recent regression, possibly introduced by #12461