`apply_member_constraints` is incomplete

Applying member constraints can result in overly strong constraints, causing the following test to fail:

fn new_defining_use<'a, F: FnOnce(T) -> &'a (), T>(_: F) {}
fn rpit<'a,  'b: 'b>(x: &'b ()) -> impl Sized + use<'a, 'b> {
    // currently a separate defining use, chooses `'static`
    new_defining_use(rpit::<'a, 'b>);
    x // this defining use needs to be `&'b ()`
}

This is caused by the following code:

https://github.com/rust-lang/rust/blob/425e142686242c7e73f5e32c79071ae266f0f355/compiler/rustc_borrowck/src/region_infer/mod.rs#L743-L759

This sort of incompleteness is unfortunately required as we can use this to lift otherwise unconstrained regions to 'static, e.g. the following snippet otherwise fails to compile

fn foo<'a, 'b>() -> impl Sized + use<'a, 'b> {
    &()
}

This may become a larger concern as we're working towards the new solver which introduces a significant amount of additional uses of opaques.

Start by reading the linked apply_member_constraints code in compiler/rustc_borrowck/src/region_infer/mod.rs around lines 743-759. Reproduce both Rust snippets and trace how defining uses choose region constraints; done means the rpit case avoids an overly strong constraint while the unconstrained-region example still does not lift regions to 'static.