change method resolution to constrain hidden types instead of rejecting method candidates

[oli-obk]

Some of these are in probes and may affect inference. This is therefore a breaking change.

This allows new code to compile on stable:

trait Trait {}

impl Trait for u32 {}

struct Bar<T>(T);

impl Bar<u32> {
    fn foo(self) {}
}

fn foo(x: bool) -> Bar<impl Sized> {
    if x {
        let x = foo(false);
        x.foo();
        //^ this used to not find the `foo` method, because while we did equate `x`'s type with possible candidates, we didn't allow opaque type inference while doing so
    }
    todo!()
}

r? @compiler-errors

fixes #121404

cc #116652