Separate private_intra_doc_links and broken_intra_doc_links into separate lints

This is not ideal because it means `deny(broken_intra_doc_links)` will
no longer `deny(private_intra_doc_links)`. However, it can't be fixed
with a new lint group, because `broken` is already in the `rustdoc` lint
group; there would need to be a way to nest groups somehow.

This also removes the early `return` so that the link will be generated
even though it gives a warning.

src/test/rustdoc-ui/intra-links-private.private.stderr

@@ -4,7 +4,7 @@ warning: public documentation for `DocMe` links to private item `DontDocMe`
 LL | /// docs [DontDocMe]
    |           ^^^^^^^^^ this item is private
    |
-   = note: `#[warn(broken_intra_doc_links)]` on by default
+   = note: `#[warn(private_intra_doc_links)]` on by default
    = note: this link resolves only because you passed `--document-private-items`, but will break without
 
 warning: 1 warning emitted

src/test/rustdoc-ui/intra-links-private.public.stderr

@@ -4,7 +4,7 @@ warning: public documentation for `DocMe` links to private item `DontDocMe`
 LL | /// docs [DontDocMe]
    |           ^^^^^^^^^ this item is private
    |
-   = note: `#[warn(broken_intra_doc_links)]` on by default
+   = note: `#[warn(private_intra_doc_links)]` on by default
    = note: this link will resolve properly if you pass `--document-private-items`
 
 warning: 1 warning emitted

src/test/rustdoc-ui/issue-74134.private.stderr

@@ -4,7 +4,7 @@ warning: public documentation for `public_item` links to private item `PrivateTy
 LL |     /// [`PrivateType`]
    |          ^^^^^^^^^^^^^ this item is private
    |
-   = note: `#[warn(broken_intra_doc_links)]` on by default
+   = note: `#[warn(private_intra_doc_links)]` on by default
    = note: this link resolves only because you passed `--document-private-items`, but will break without
 
 warning: 1 warning emitted

src/test/rustdoc-ui/issue-74134.public.stderr

@@ -4,7 +4,7 @@ warning: public documentation for `public_item` links to private item `PrivateTy
 LL |     /// [`PrivateType`]
    |          ^^^^^^^^^^^^^ this item is private
    |
-   = note: `#[warn(broken_intra_doc_links)]` on by default
+   = note: `#[warn(private_intra_doc_links)]` on by default
    = note: this link will resolve properly if you pass `--document-private-items`
 
 warning: 1 warning emitted

src/test/rustdoc/intra-doc-link-private.rs

@@ -0,0 +1,6 @@
+#![crate_name = "private"]
+// compile-flags: --document-private-items
+/// docs [DontDocMe]
+// @has private/struct.DocMe.html '//*a[@href="../private/struct.DontDocMe.html"]' 'DontDocMe'
+pub struct DocMe;
+struct DontDocMe;