Rollup merge of #104595 - compiler-errors:poly-existential-predicate, r=lcnr

Add `PolyExistentialPredicate` type alias

Wrapping `ExistentialPredicate`s in a binder is very common, and this alias already exists for the `PolyExistential{TraitRef,Projection}` types.

compiler/rustc_middle/src/ty/error.rs

@@ -69,9 +69,7 @@ pub enum TypeError<'tcx> {
     CyclicTy(Ty<'tcx>),
     CyclicConst(ty::Const<'tcx>),
     ProjectionMismatched(ExpectedFound<DefId>),
-    ExistentialMismatch(
-        ExpectedFound<&'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>>,
-    ),
+    ExistentialMismatch(ExpectedFound<&'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>>),
     ObjectUnsafeCoercion(DefId),
     ConstMismatch(ExpectedFound<ty::Const<'tcx>>),