Replace __cplusplus comparisons and dialect __has_feature checks with TEST_STD_VER.

This is a huge cleanup that helps make the libc++ test suite more portable.
Patch from STL@microsoft.com. Thanks STL!


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@272716 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/iterators/iterator.container/empty.pass.cpp

@@ -12,7 +12,9 @@
 // template <class T, size_t N> constexpr bool empty(const T (&array)[N]) noexcept;  // C++17
 // template <class E> constexpr bool empty(initializer_list<E> il) noexcept;         // C++17
 
-#if __cplusplus <= 201402L
+#include "test_macros.h"
+
+#if TEST_STD_VER <= 14
 int main () {}
 #else