[libc++][NFC] Rename _EnableIf to __enable_if_t for consistency

In other places in the code, we use lowercase spelling for things that
are not available in prior standards.

Differential Revision: https://reviews.llvm.org/D109435

NOKEYCHECK=True
GitOrigin-RevId: b4e88d4db12e9460e581de453c8603eb280f145b
diff --git a/include/bitset b/include/bitset
index 4b8827e..f9549e8 100644
--- a/include/bitset
+++ b/include/bitset
@@ -679,7 +679,7 @@
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR bitset() _NOEXCEPT {}
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
         bitset(unsigned long long __v) _NOEXCEPT : base(__v) {}
-    template<class _CharT, class = _EnableIf<_IsCharLikeType<_CharT>::value> >
+    template<class _CharT, class = __enable_if_t<_IsCharLikeType<_CharT>::value> >
         explicit bitset(const _CharT* __str,
                         typename basic_string<_CharT>::size_type __n = basic_string<_CharT>::npos,
                         _CharT __zero = _CharT('0'), _CharT __one = _CharT('1'));