[libc++][NFC] Replace enable_if with __enable_if_t in a few places

Reviewed By: ldionne, #libc

Spies: jloser, libcxx-commits

Differential Revision: https://reviews.llvm.org/D128400

NOKEYCHECK=True
GitOrigin-RevId: 4887d047a31fa89735f36ae03b697abbbd71f0dc
diff --git a/include/array b/include/array
index e96c3d8..867dd6b 100644
--- a/include/array
+++ b/include/array
@@ -438,12 +438,7 @@
 
 template <class _Tp, size_t _Size>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-typename enable_if
-<
-    _Size == 0 ||
-    __is_swappable<_Tp>::value,
-    void
->::type
+__enable_if_t<_Size == 0 || __is_swappable<_Tp>::value, void>
 swap(array<_Tp, _Size>& __x, array<_Tp, _Size>& __y)
                                   _NOEXCEPT_(noexcept(__x.swap(__y)))
 {