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/language.support/support.runtime/cstdarg.pass.cpp

@@ -11,11 +11,13 @@
 
 #include <cstdarg>
 
+#include "test_macros.h"
+
 #ifndef va_arg
 #error va_arg not defined
 #endif
 
-#if __cplusplus >= 201103L
+#if TEST_STD_VER >= 11
 #  ifndef va_copy
 #    error va_copy is not defined when c++ >= 11
 #  endif