Support all variants of WherePredicate

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