Fix inserting of trait items from trait constraints into namespace

[ironcev]

Type checking of trait constraints inserts items of traits listed in the constraints, into the namespace.

E.g., the following project compiles without errors, because C gets inserted into the namespace of f.

library;

pub trait A {
    const C: bool;
}

struct S<T> { }

impl<T> S<T> {
    fn f() -> bool where T: A {
        C // <<<--- NO ERROR HERE!!
    }
}

Additionally, this insertion causes issues like #7396.