add additional `TypeFlags` fast paths Some crates, e.g. `diesel`, have items with a lot of where-clauses (more than 150). In these cases checking the `TypeFlags` of the whole `param_env` can be very beneficial. This adds `fn fold_clauses` to mirror the existing `fn visit_clauses` and then uses this in folders which fold `ParamEnv`s. Split out from rust-lang/rust#141451, depends on rust-lang/rust#141442. r? `@compiler-errors`