IAT: Reject implicit lifetime bounds of trait object types passed to inherent projections as indeterminate in ItemCtxts

[fmease]

This is for future-proofing / an extension of my efforts to make object lifetime defaulting more predictable & consistent across generic containers (see also #129543).

Examples that currently compile but ought not to:

#![feature(inherent_associated_types)]

struct Self0;
impl Self0 { type Project<'a, T: 'a + ?Sized> = (); }

type Demo0<'r> = Self0::Project<'r, dyn Trait>;
//                                  ^^^^^^^^^ ought to be rejected

struct Self1<'a>(&'a ());
impl<'a> Self1<'a> { type Project<T: 'a + ?Sized> = (); }

type Demo1<'r> = Self1<'r>::Project<dyn Trait>;
//                                  ^^^^^^^^^ ought to be rejected

trait Trait {}

Note that this shouldn't/doesn't affect cases where the trait object types has own bounds that can be used as they take precedence (which The Reference gets wrong, cc rust-lang/reference#1407). Example of something that isn't affected:

#![feature(inherent_associated_types)]

trait Trait: 'static {}
//           ^^^^^^^ (!)

struct Zelf;
impl Zelf { type Project<'a, T: 'a + ?Sized> = (); }

type Demo0<'r> = Zelf::Project<'r, dyn Trait>;
//                                 ^^^^^^^^^ OK!
//                                           This elaborates to `dyn Trait + 'static`
//                                           due to the bound on `Trait` which takes
//                                           precedence over "`named_bound_var`".

---

We need to set the ambient object lifetime default for inherent projections to None in resolve_bound_vars, this way it'll be rejected later in ItemCtxts during HIR ty lowering.

"Problem" (the crux of the matter, actually): We don't have the resolution of TypeRelative paths in RBV, so arguably we should just do that for all unknown¹ TypeRelative paths. I tried that already in #129543 and crater came back clean! I might just fix this issue in #129543 come to think about it.

Footnotes

1. https://github.com/rust-lang/rust/pull/129543#issuecomment-2872926835 ↩

[fmease]

Blocking this on #129543. Edit: I might just fix it in that PR (the first commit already fixes it but the latest commit undoes it to experiment with ad hoc resolution of trait associated types in RBV).