rustdoc: Allow linking from private items to private types

Fixes #74134

After PR #72771 this would trigger an intra_doc_link_resolution_failure warning
when rustdoc is invoked without --document-private-items. Links from private
items to private types are however never actually generated in that case and
thus shouldn't produce a warning. These links are in fact a very useful tool to
document crate internals.

Tests are added for all 4 combinations of public/private items and link
targets. Test 1 is the case mentioned above and fails without this commit. Tests
2 - 4 passed before already but are added nonetheless to prevent regressions.

src/test/rustdoc/issue-74134-1.rs

@@ -0,0 +1,10 @@
+#![deny(intra_doc_link_resolution_failure)]
+
+// Linking from a private item to a private type is fine without --document-private-items.
+
+struct Private;
+
+pub struct Public {
+    /// [`Private`]
+    private: Private,
+}

src/test/rustdoc/issue-74134-2.rs

@@ -0,0 +1,11 @@
+// compile-flags: --document-private-items
+#![deny(intra_doc_link_resolution_failure)]
+
+// Linking from a private item to a private type is fine with --document-private-items.
+
+struct Private;
+
+pub struct Public {
+    /// [`Private`]
+    private: Private,
+}

src/test/rustdoc/issue-74134-3.rs

@@ -0,0 +1,11 @@
+// should-fail
+#![deny(intra_doc_link_resolution_failure)]
+
+// Linking from a public item to a private type fails without --document-private-items.
+
+struct Private;
+
+pub struct Public {
+    /// [`Private`]
+    pub public: u32,
+}

src/test/rustdoc/issue-74134-4.rs

@@ -0,0 +1,11 @@
+// compile-flags: --document-private-items
+#![deny(intra_doc_link_resolution_failure)]
+
+// Linking from a public item to a private type is fine with --document-private-items.
+
+struct Private;
+
+pub struct Public {
+    /// [`Private`]
+    pub public: u32,
+}