auto merge of #20180 : jroesch/rust/clean-where-predicate, r=alexcrichton

Add support for all variants of ast::WherePredicate in clean/mod.rs. Fixes #20048, but will need modification when EqualityPredicates are fully implemented in #20041.

diff --cc src/librustdoc/clean/mod.rs
Simple merge

diff --cc src/librustdoc/html/format.rs
Simple merge