apply_member_constraints: fix placeholder check

[lcnr]

Checking whether the member region is an existential region from a higher universe is just wrong and I am pretty sure we've added that check by accident as the naming was just horribly confusing before #140466.

I've encountered this issue separately while working on #139587, but feel like it's probably easier to separately FCP this change. This allows the following code to compile

trait Proj<'a> {
    type Assoc;
}
impl<'a, 'b, F: FnOnce() -> &'b ()> Proj<'a> for F {
    type Assoc = ();
}

fn is_proj<F: for<'a> Proj<'a>>(f: F) {}
fn define<'a>() -> impl Sized + use<'a> {
    // This adds a use of `opaque::<'a>` with hidden type `&'unconstrained_b ()`.
    // 'unconstrained_b is an inference variable from a higher universe as it gets
    // created inside of the binder of `F: for<'a> Proj<'a>`. This previously
    // caused us to not apply member constraints. We now do, constraining
    // it to `'a`.
    is_proj(define::<'a>);
    &()
}

fn main() {}

This should not be breaking change, even in theory. Applying member constraints is incomplete in rare circumstances which means that applying them in more cases can cause spurious errors, cc #140569/#142073. However, as we always skipped these member regions in apply_member_constraints the skipped region is guaranteed to cause an error in check_member_constraints later on.

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[lcnr]

@rfcbot fcp merge

[rfcbot]

Team member @lcnr has proposed to merge this. The next step is review by the rest of the tagged team members:

• @BoxyUwU
• @compiler-errors
• @jackh726
• @lcnr
• @nikomatsakis
• @oli-obk
• @spastorino

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