Fix linkcheck issues

Most of these are because alloc uses `#[lang_item]` to define methods,
but core documents primitives before those methods are available.

- Fix rustdoc-js-std test

  For some reason this change made CStr not show up in the results for
  `str,u8`. Since it still shows up for str, and since it wasn't a great
  match for that query anyway, I think this is ok to let slide.

- Add test that all primitives can be linked to
- Enable `doc(primitive)` in `core` as well
- Add linkcheck exception specifically for Windows

  Ideally this would be done automatically by the linkchecker by
  replacing `\\` with forward slashes, but this PR is already a ton of
  work ...

- Don't forcibly fail linkchecking if there's a broken intra-doc link on Windows

  Previously, it would exit with a hard error if a missing file had `::`
  in it. This changes it to report a missing file instead, which allows
  adding an exception.

Author: jyn514

core/src/char/methods.rs

@@ -24,7 +24,7 @@ impl char {
     /// decoding error.
     ///
     /// It can occur, for example, when giving ill-formed UTF-8 bytes to
-    /// [`String::from_utf8_lossy`](string/struct.String.html#method.from_utf8_lossy).
+    /// [`String::from_utf8_lossy`](../std/string/struct.String.html#method.from_utf8_lossy).
     #[stable(feature = "assoc_char_consts", since = "1.52.0")]
     pub const REPLACEMENT_CHARACTER: char = '\u{FFFD}';
 

core/src/slice/mod.rs

@@ -2257,9 +2257,9 @@ impl<T> [T] {
     /// assert!(match r { Ok(1..=4) => true, _ => false, });
     /// ```
     // Lint rustdoc::broken_intra_doc_links is allowed as `slice::sort_by_key` is
-    // in crate `alloc`, and as such doesn't exists yet when building `core`.
-    // links to downstream crate: #74481. Since primitives are only documented in
-    // libstd (#73423), this never leads to broken links in practice.
+    // in crate `alloc`, and as such doesn't exists yet when building `core`: #74481.
+    // This breaks links when slice is displayed in core, but changing it to use relative links
+    // would break when the item is re-exported. So allow the core links to be broken for now.
     #[allow(rustdoc::broken_intra_doc_links)]
     #[stable(feature = "slice_binary_search_by_key", since = "1.10.0")]
     #[inline]