[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/stack b/include/stack
index 79cc15d..436993c 100644
--- a/include/stack
+++ b/include/stack
@@ -158,28 +158,28 @@
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         explicit stack(const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0)
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0)
             : c(__a) {}
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         stack(const container_type& __c, const _Alloc& __a,
-              _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0)
+              __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0)
             : c(__c, __a) {}
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         stack(const stack& __s, const _Alloc& __a,
-              _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0)
+              __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0)
             : c(__s.c, __a) {}
 #ifndef _LIBCPP_CXX03_LANG
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         stack(container_type&& __c, const _Alloc& __a,
-              _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0)
+              __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0)
             : c(_VSTD::move(__c), __a) {}
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         stack(stack&& __s, const _Alloc& __a,
-              _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0)
+              __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0)
             : c(_VSTD::move(__s.c), __a) {}
 #endif // _LIBCPP_CXX03_LANG
 
@@ -297,7 +297,7 @@
 
 template <class _Tp, class _Container>
 inline _LIBCPP_INLINE_VISIBILITY
-_EnableIf<__is_swappable<_Container>::value, void>
+__enable_if_t<__is_swappable<_Container>::value, void>
 swap(stack<_Tp, _Container>& __x, stack<_Tp, _Container>& __y)
     _NOEXCEPT_(_NOEXCEPT_(__x.swap(__y)))
 {