Fix ICE in rustdoc when impl is nested in a func

[GoldsteinE]

For cases like

fn foo() {
    impl<E> Trait for Bar<E>
    where E: Baz {}
}

hir_enclosing_body_owner() in rustdoc considered fn foo() to be the “enclosing body” of E, which caused to to produce HirIds for E with owner set to fn foo() instead of the impl. This instead treats E as having no enclosing body.

r? @GuillaumeGomez as the author of this logic, I’m not 100% I understood it correctly.

Fixes #147882.
Fixes #147057.

[GuillaumeGomez]

I'll side step here and let @fmease review this as they're the expert of this topic.

r? fmease

[rustbot]

fmease is currently at their maximum review capacity.
They may take a while to respond.

[fmease]

Haven't looked at the impl yet; I guess this also fixes #147057 cuz #147882 is likely a dupe of the former?

[GoldsteinE]

I’ve checked this, and yes, it fixes #147057. Updated the top post accordingly.