Auto merge of #79322 - jyn514:refactor-impl, r=estebank

Separate out a `hir::Impl` struct

This makes it possible to pass the `Impl` directly to functions, instead
of having to pass each of the many fields one at a time. It also
simplifies matches in many cases.

See `rustc_save_analysis::dump_visitor::process_impl` or `rustdoc::clean::clean_impl` for a good example of how this makes `impl`s easier to work with.

r? `@petrochenkov` maybe?

compiler/rustc_middle/src/hir/map/mod.rs

@@ -505,7 +505,7 @@ impl<'hir> Map<'hir> {
                     | ItemKind::Union(_, generics)
                     | ItemKind::Trait(_, _, generics, ..)
                     | ItemKind::TraitAlias(generics, _)
-                    | ItemKind::Impl { generics, .. },
+                    | ItemKind::Impl(Impl { generics, .. }),
                 ..
             }) => Some(generics),
             _ => None,