auto merge of #12195 : kballard/rust/rustdoc-strip-impls-of-stripped, r=cmr

Strip trait impls for types that are stripped either due to the strip-hidden or strip-private passes.

This fixes the search index including trait methods on stripped structs (which breaks searching), and it also removes private types from the implementors list of a trait.

Fixes #9981 and #11439.