[Libc++] Use #pragma push_macro/pop_macro to better handle min/max on Windows

Summary:
This patch improves how libc++ handles min/max macros within the headers. Previously libc++ would undef them and emit a warning.
This patch changes libc++ to use `#pragma push_macro`  to save the macro before undefining it, and `#pragma pop_macro` to restore the macros and the end of the header.

Reviewers: mclow.lists, bcraig, compnerd, EricWF

Reviewed By: EricWF

Subscribers: cfe-commits, krytarowski

Differential Revision: https://reviews.llvm.org/D33080

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@304357 91177308-0d34-0410-b5e6-96231b3b80d8

include/bitset

@@ -113,10 +113,6 @@ template <size_t N> struct hash<std::bitset<N>>;
 
 */
 
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#pragma GCC system_header
-#endif
-
 #include <__config>
 #include <__bit_reference>
 #include <cstddef>
@@ -126,7 +122,13 @@ template <size_t N> struct hash<std::bitset<N>>;
 #include <iosfwd>
 #include <__functional_base>
 
-#include <__undef_min_max>
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
@@ -1090,4 +1092,6 @@ operator<<(basic_ostream<_CharT, _Traits>& __os, const bitset<_Size>& __x);
 
 _LIBCPP_END_NAMESPACE_STD
 
+_LIBCPP_POP_MACROS
+
 #endif  // _LIBCPP_BITSET