rustdoc: Keep hidden struct fields out of search

Previously, private and `#[doc(hidden)]` struct fields appeared in the
search index despite being hidden from the struct's documentation.

Fix #15490
auto merge of #16016 : tomjakubowski/rust/rustdoc-fix-15490, r=alexcrichton

Previously, private and `#[doc(hidden)]` struct fields appeared in the
search index despite being hidden from the struct's documentation.

Fix #15490