Normalize associated types in paths in expressions

[lowr]

Part of #14393

When we resolve paths in expressions (either path expressions or paths in struct expressions), there's a need of projection normalization, which TyLoweringContext cannot do on its own. We've been properly applying normalization for paths in struct expressions without type anchor, but not for others:

enum E {
    S { v: i32 }
    Empty,
}

impl Foo for Bar {
    type Assoc = E;
    fn foo() {
        let _ = Self::Assoc::S { v: 42 };   // path in struct expr without type anchor; we already support this
        let _ = <Self>::Assoc::S { v: 42 }; // path in struct expr with type anchor; resolves with this PR
        let _ = Self::Assoc::Empty;         // path expr; resolves with this PR
    }
}

With this PR we correctly resolve the whole path, but we need some more tweaks in HIR and/or IDE layers to properly resolve a qualifier (prefix) of such paths and provide IDE features that are pointed out in #14393 to be currently broken.

[HKalbasi]

There are a bunch of unwrap in this function, which I think it is possible to remove by defining some windows like thing for PathSegments (or by using Itertools::tuple_windows) and a function which goes from a &PathSegment to a &PathSegments (similar to slice::from_ref).

[Veykril]

we've got these path segment unwraps in more places (our API for paths isn't really that great currently)

[HKalbasi]

Okay, so we can do it in a follow up.

[HKalbasi]

@bors r+

[bors]

📌 Commit 6447d48 has been approved by HKalbasi

It is now in the queue for this repository.

[bors]

⌛ Testing commit 6447d48 with merge af30656...

[bors]

☀️ Test successful - checks-actions
Approved by: HKalbasi
Pushing af30656 to master...