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/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