Distinguish between private items and hidden items in rustdoc

I believe rustdoc should not be conflating private items (visibility
lower than `pub`) and hidden items (attribute `doc(hidden)`). This
matters now that Cargo is passing --document-private-items by default
for bin crates. In bin crates that rely on macros, intentionally hidden
implementation details of the macros can overwhelm the actual useful
internal API that one would want to document.

This PR restores the strip-hidden pass when documenting private items,
and introduces a separate unstable --document-hidden-items option to
skip the strip-hidden pass. The two options are orthogonal to one
another.

src/test/rustdoc/issue-46380.rs

@@ -1,5 +0,0 @@
-// compile-flags: --document-private-items
-
-// @has issue_46380/struct.Hidden.html
-#[doc(hidden)]
-pub struct Hidden;

src/test/rustdoc/issue-67851-both.rs

@@ -0,0 +1,8 @@
+// compile-flags: -Zunstable-options --document-private-items --document-hidden-items
+
+// @has issue_67851_both/struct.Hidden.html
+#[doc(hidden)]
+pub struct Hidden;
+
+// @has issue_67851_both/struct.Private.html
+struct Private;

src/test/rustdoc/issue-67851-hidden.rs

@@ -0,0 +1,8 @@
+// compile-flags: -Zunstable-options --document-hidden-items
+
+// @has issue_67851_hidden/struct.Hidden.html
+#[doc(hidden)]
+pub struct Hidden;
+
+// @!has issue_67851_hidden/struct.Private.html
+struct Private;

src/test/rustdoc/issue-67851-neither.rs

@@ -0,0 +1,6 @@
+// @!has issue_67851_neither/struct.Hidden.html
+#[doc(hidden)]
+pub struct Hidden;
+
+// @!has issue_67851_neither/struct.Private.html
+struct Private;

src/test/rustdoc/issue-67851-private.rs

@@ -0,0 +1,8 @@
+// compile-flags: --document-private-items
+
+// @!has issue_67851_private/struct.Hidden.html
+#[doc(hidden)]
+pub struct Hidden;
+
+// @has issue_67851_private/struct.Private.html
+struct Private;