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_trait_selection/src/solve/project_goals.rs

@@ -107,7 +107,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
     fn probe_and_match_goal_against_assumption(
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,
-        assumption: ty::Binder<'tcx, ty::Clause<'tcx>>,
+        assumption: ty::Clause<'tcx>,
         then: impl FnOnce(&mut EvalCtxt<'_, 'tcx>) -> QueryResult<'tcx>,
     ) -> QueryResult<'tcx> {
         if let Some(projection_pred) = assumption.as_projection_clause() {