Rollup merge of #112772 - compiler-errors:clauses-1, r=lcnr

Add a fully fledged `Clause` type, rename old `Clause` to `ClauseKind`

Does two basic things before I put up a more delicate set of PRs (along the lines of #112714, but hopefully much cleaner) that migrate existing usages of `ty::Predicate` to `ty::Clause` (`predicates_of`/`item_bounds`/`ParamEnv::caller_bounds`).

1. Rename `Clause` to `ClauseKind`, so it's parallel with `PredicateKind`.
2. Add a new `Clause` type which is parallel to `Predicate`.
    * This type exposes `Clause::kind(self) -> Binder<'tcx, ClauseKind<'tcx>>` which is parallel to `Predicate::kind` 😸

The new `Clause` type essentially acts as a newtype wrapper around `Predicate` that asserts that it is specifically a `PredicateKind::Clause`. Turns out from experimentation[^1] that this is not negative performance-wise, which is wonderful, since this a much simpler design than something that requires encoding the discriminant into the alignment bits of a predicate kind, or something else like that...

r? ``@lcnr`` or ``@oli-obk``

[^1]: https://github.com/rust-lang/rust/pull/112714#issuecomment-1595653910

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -376,7 +376,7 @@ define_tables! {
     attr_flags: Table<DefIndex, AttrFlags>,
     def_path_hashes: Table<DefIndex, DefPathHash>,
     explicit_item_bounds: Table<DefIndex, LazyArray<(ty::Predicate<'static>, Span)>>,
-    inferred_outlives_of: Table<DefIndex, LazyArray<(ty::Clause<'static>, Span)>>,
+    inferred_outlives_of: Table<DefIndex, LazyArray<(ty::ClauseKind<'static>, Span)>>,
     inherent_impls: Table<DefIndex, LazyArray<DefIndex>>,
     associated_types_for_impl_traits_in_associated_fn: Table<DefIndex, LazyArray<DefId>>,
     opt_rpitit_info: Table<DefIndex, Option<LazyValue<ty::ImplTraitInTraitData>>>,