Fix intra-doc links for Self on primitives

- Remove the difference between `parent_item` and `current_item`; these
  should never have been different.
- Remove `current_item` from `resolve` and `variant_field` so that
  `Self` is only substituted in one place at the very start.
- Resolve the current item as a `DefId`, not a `HirId`. This is what
  actually fixed the bug.

Hacks:
- `clean` uses `TypedefItem` when it _really_ should be
  `AssociatedTypeItem`. I tried fixing this without success and hacked
  around it instead (see comments)
- This stringifies DefIds, then resolves them a second time. This is
  really silly and rustdoc should just use DefIds throughout. Fixing
  this is a larger task than I want to take on right now.

src/test/rustdoc/intra-link-prim-self.rs

@@ -0,0 +1,36 @@
+// ignore-tidy-linelength
+#![deny(broken_intra_doc_links)]
+#![feature(lang_items)]
+#![feature(no_core)]
+#![no_core]
+
+#[lang = "usize"]
+/// [Self::f]
+/// [Self::MAX]
+// @has intra_link_prim_self/primitive.usize.html
+// @has - '//a[@href="https://doc.rust-lang.org/nightly/std/primitive.usize.html#method.f"]' 'Self::f'
+// @has - '//a[@href="https://doc.rust-lang.org/nightly/std/primitive.usize.html#associatedconstant.MAX"]' 'Self::MAX'
+impl usize {
+    /// Some docs
+    pub fn f() {}
+
+    /// 10 and 2^32 are basically the same.
+    pub const MAX: usize = 10;
+
+    // FIXME(#8995) uncomment this when associated types in inherent impls are supported
+    // @ has - '//a[@href="https://doc.rust-lang.org/nightly/std/primitive.usize.html#associatedtype.ME"]' 'Self::ME'
+    // / [Self::ME]
+    //pub type ME = usize;
+}
+
+#[doc(primitive = "usize")]
+/// This has some docs.
+mod usize {}
+
+/// [S::f]
+/// [Self::f]
+pub struct S;
+
+impl S {
+    pub fn f() {}
+}