Fix min/max usage in variant

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@318622 91177308-0d34-0410-b5e6-96231b3b80d8

include/variant

@@ -213,6 +213,9 @@ namespace std {
 #pragma GCC system_header
 #endif
 
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
 namespace std { // explicitly not using versioning namespace
 
 class _LIBCPP_EXCEPTION_ABI bad_variant_access : public exception {
@@ -1586,4 +1589,6 @@ struct _LIBCPP_TEMPLATE_VIS hash<monostate> {
 
 _LIBCPP_END_NAMESPACE_STD
 
+_LIBCPP_POP_MACROS
+
 #endif  // _LIBCPP_VARIANT