Fix ill-formed primary template in P0896R4_ranges_subrange (#3120)

* Fix ill-formed primary template in P0896R4_ranges_subrange

The intent is that `illformed<T>()` be ill-formed so the primary `kind_of` template fails instantiation, but unfortunately it's ill-formed with no diagnostic required since no specialization of this template could be valid. (Unqualified lookup for `illformed` finds nothing, and ADL can find nothing since there are no arguments and therefore no associated namespaces.)

Reported internally by the FE team.

Mirrors the STL changes in MSVC-PR-425776.

tests/std/tests/P0896R4_ranges_subrange/test.compile.pass.cpp

@@ -807,7 +807,12 @@ namespace test_subrange {
     inline constexpr bool is_subrange<subrange<I, S, K>> = true;
 
     template <class T>
-    inline constexpr auto kind_of = illformed<T>();
+    struct illformed {
+        static_assert(always_false<T>);
+    };
+
+    template <class T>
+    inline constexpr auto kind_of = illformed<T>{};
     template <class I, class S, subrange_kind K>
     inline constexpr auto kind_of<subrange<I, S, K>> = K;