[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_view

@@ -166,7 +166,6 @@ namespace std {
 */
 
 #include <__config>
-
 #include <__string>
 #include <algorithm>
 #include <iterator>
@@ -178,6 +177,10 @@ namespace std {
 #pragma GCC system_header
 #endif
 
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template<class _CharT, class _Traits = char_traits<_CharT> >
@@ -784,4 +787,6 @@ inline namespace literals
 #endif
 _LIBCPP_END_NAMESPACE_STD
 
+_LIBCPP_POP_MACROS
+
 #endif // _LIBCPP_STRING_VIEW