[libc++][NFC] Move __libcpp_is_integral into the else branch (#142556)

This makes it clear that `__libcpp_is_integral` is an implementation
detail of `is_integral` if we don't have `__is_integral` and not its own
utility.

Author: philnik777

libcxx/include/__type_traits/is_integral.h

@@ -19,6 +19,18 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+#if __has_builtin(__is_integral)
+
+template <class _Tp>
+struct _LIBCPP_NO_SPECIALIZATIONS is_integral : _BoolConstant<__is_integral(_Tp)> {};
+
+#  if _LIBCPP_STD_VER >= 17
+template <class _Tp>
+_LIBCPP_NO_SPECIALIZATIONS inline constexpr bool is_integral_v = __is_integral(_Tp);
+#  endif
+
+#else
+
 // clang-format off
 template <class _Tp> struct __libcpp_is_integral                     { enum { value = 0 }; };
 template <>          struct __libcpp_is_integral<bool>               { enum { value = 1 }; };
@@ -47,18 +59,6 @@ template <>          struct __libcpp_is_integral<__uint128_t>        { enum { va
 #endif
 // clang-format on
 
-#if __has_builtin(__is_integral)
-
-template <class _Tp>
-struct _LIBCPP_NO_SPECIALIZATIONS is_integral : _BoolConstant<__is_integral(_Tp)> {};
-
-#  if _LIBCPP_STD_VER >= 17
-template <class _Tp>
-_LIBCPP_NO_SPECIALIZATIONS inline constexpr bool is_integral_v = __is_integral(_Tp);
-#  endif
-
-#else
-
 template <class _Tp>
 struct is_integral : public _BoolConstant<__libcpp_is_integral<__remove_cv_t<_Tp> >::value> {};