[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/string

@@ -484,14 +484,16 @@ basic_string<char32_t> operator "" s( const char32_t *str, size_t len ); // C++1
 #include <cstdint>
 #endif
 
-#include <__undef_min_max>
-
 #include <__debug>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 // fpos
@@ -4041,4 +4043,6 @@ _LIBCPP_EXTERN_TEMPLATE(string operator+<char, char_traits<char>, allocator<char
 
 _LIBCPP_END_NAMESPACE_STD
 
+_LIBCPP_POP_MACROS
+
 #endif  // _LIBCPP_STRING