Replace mk_foo calls with infer_foo where possible.

There are several `mk_foo`/`intern_foo` pairs, where the former takes an
iterator and the latter takes a slice. (This naming convention is bad,
but that's a fix for another PR.)

This commit changes several `mk_foo` occurrences into `intern_foo`,
avoiding the need for some `.iter()`/`.into_iter()` calls. Affected
cases:
- mk_type_list
- mk_tup
- mk_substs
- mk_const_list

compiler/rustc_ty_utils/src/consts.rs

@@ -144,7 +144,7 @@ fn recurse_build<'tcx>(
             for &id in args.iter() {
                 new_args.push(recurse_build(tcx, body, id, root_span)?);
             }
-            let new_args = tcx.mk_const_list(new_args.iter());
+            let new_args = tcx.intern_const_list(&new_args);
             tcx.mk_const(Expr::FunctionCall(fun, new_args), node.ty)
         }
         &ExprKind::Binary { op, lhs, rhs } if check_binop(op) => {