Walter pointed out some missing includes in the tests. Fixing the includes uncovered a couple bugs in the _v type traits. Fixed those, too

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@252612 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/std/utilities/meta/meta.unary/meta.unary.cat/is_class.pass.cpp b/test/std/utilities/meta/meta.unary/meta.unary.cat/is_class.pass.cpp
index d720791..97671e7 100644
--- a/test/std/utilities/meta/meta.unary/meta.unary.cat/is_class.pass.cpp
+++ b/test/std/utilities/meta/meta.unary/meta.unary.cat/is_class.pass.cpp
@@ -12,6 +12,7 @@
 // is_class
 
 #include <type_traits>
+#include <cstddef>        // for std::nullptr_t
 #include "test_macros.h"
 
 template <class T>