Unify primitive errors with other intra-link errors

Now that `PrimTy::name()` exists, there's no need to carry around the
name of the primitive that failed to resolve. This removes the variants
special-casing primitives in favor of `NotResolved`.

- Remove `NoPrimitiveImpl` and `NoPrimitiveAssocItem`
- Remove hacky `has_primitive` check in `resolution_failure()`
- Fixup a couple tests that I forgot to `--bless` before

src/test/rustdoc-ui/intra-link-errors.rs

@@ -54,7 +54,16 @@
 
 /// [u8::not_found]
 //~^ ERROR unresolved link
-//~| NOTE the builtin type `u8` does not have an associated item named `not_found`
+//~| NOTE the builtin type `u8` has no associated item named `not_found`
+
+/// [std::primitive::u8::not_found]
+//~^ ERROR unresolved link
+//~| NOTE the builtin type `u8` has no associated item named `not_found`
+
+/// [type@Vec::into_iter]
+//~^ ERROR unresolved link
+//~| HELP to link to the associated function, add parentheses
+//~| NOTE this link resolves to the associated function `into_iter`
 
 /// [S!]
 //~^ ERROR unresolved link