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/std/numerics/c.math/cmath.pass.cpp b/libcxx/test/std/numerics/c.math/cmath.pass.cpp
index 5fe1c42..7161bd3 100644
--- a/libcxx/test/std/numerics/c.math/cmath.pass.cpp
+++ b/libcxx/test/std/numerics/c.math/cmath.pass.cpp
@@ -13,6 +13,7 @@
 #include <type_traits>
 #include <cassert>
 
+#include "test_macros.h"
 #include "hexfloat.h"
 
 // convertible to int/float/double/etc