Remaining unsoundness in the termination check

[arnaudgolfouse]

After #1220, the termination check must be reworked (again), as it allows the following two incorrect examples:

Recursive instantiation

trait Tr1 {
    fn f();
}

trait Tr2 {
    fn g();
}

impl<T> Tr1 for T where T : Tr2 {
    fn f() {
        T::g()
    }
}

impl Tr2 for u32 {
    fn g() {
        h()
    }
}

fn call_f<T: Tr1>() {
    T::f()
}

fn h() {
    call_f::<u32>()
}

When calling call_f, the bound T: Tr2 does not appear, but it is used. To visit trait bounds correctly, see in rustc_traits/src/codegen.rs, the implementation of function codegen_select_candidate.

Default function uses trait instance

trait Tr {
    fn f();
}

default impl<T> Tr for T {
    fn f() { g::<Self>(); }
}

fn g<T: Tr>() {
    <T as Tr>::f();
}

This is never flagged by the current check, even if we do something like

impl Tr for i32 {}

fn h() {
    <i32 as Tr>::f();
}

We should forbid f from using the trait instance it is defining.