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!

llvm-svn: 272716
diff --git a/libcxx/test/support/MoveOnly.h b/libcxx/test/support/MoveOnly.h
index ee6ae7c..a3e9cca 100644
--- a/libcxx/test/support/MoveOnly.h
+++ b/libcxx/test/support/MoveOnly.h
@@ -10,6 +10,8 @@
 #ifndef MOVEONLY_H
 #define MOVEONLY_H
 
+#include "test_macros.h"
+
 #ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
 
 #include <cstddef>