[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/__threading_support

@@ -30,9 +30,12 @@
 #include <windows.h>
 #include <process.h>
 #include <fibersapi.h>
-#include <__undef_min_max>
 #endif
 
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+
 #if defined(_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL) || \
     defined(_LIBCPP_BUILDING_THREAD_LIBRARY_EXTERNAL)
 #define _LIBCPP_THREAD_ABI_VISIBILITY _LIBCPP_FUNC_VIS
@@ -629,6 +632,8 @@ int __libcpp_tls_set(__libcpp_tls_key __key, void *__p)
 
 _LIBCPP_END_NAMESPACE_STD
 
+_LIBCPP_POP_MACROS
+
 #endif // !_LIBCPP_HAS_NO_THREADS
 
 #endif // _LIBCPP_THREADING_SUPPORT