Recognize private_intra_doc_links as a lint

Previously, trying to allow this would give another error!

```
warning: unknown lint: `private_intra_doc_links`
 --> private.rs:1:10
  |
1 | #![allow(private_intra_doc_links)]
  |          ^^^^^^^^^^^^^^^^^^^^^^^ help: did you mean: `broken_intra_doc_links`
  |
  = note: `#[warn(unknown_lints)]` on by default

warning: public documentation for `DocMe` links to private item `DontDocMe`
 --> private.rs:2:11
  |
2 | /// docs [DontDocMe]
  |           ^^^^^^^^^ this item is private
  |
  = note: `#[warn(private_intra_doc_links)]` on by default
  = note: this link will resolve properly if you pass `--document-private-items`
```

compiler/rustc_lint_defs/src/builtin.rs

@@ -2790,6 +2790,7 @@ declare_lint_pass! {
         UNSTABLE_NAME_COLLISIONS,
         IRREFUTABLE_LET_PATTERNS,
         BROKEN_INTRA_DOC_LINKS,
+        PRIVATE_INTRA_DOC_LINKS,
         INVALID_CODEBLOCK_ATTRIBUTES,
         MISSING_CRATE_LEVEL_DOCS,
         MISSING_DOC_CODE_EXAMPLES,