Add term to ExistentialProjection

Also prevent ICE when adding a const in associated const equality.

compiler/rustc_symbol_mangling/src/v0.rs

@@ -559,7 +559,10 @@ impl<'tcx> Printer<'tcx> for &mut SymbolMangler<'tcx> {
                         let name = cx.tcx.associated_item(projection.item_def_id).ident;
                         cx.push("p");
                         cx.push_ident(name.as_str());
-                        cx = projection.ty.print(cx)?;
+                        cx = match projection.term {
+                            ty::Term::Ty(ty) => ty.print(cx),
+                            ty::Term::Const(c) => c.print(cx),
+                        }?;
                     }
                     ty::ExistentialPredicate::AutoTrait(def_id) => {
                         cx = cx.print_def_path(*def_id, &[])?;