[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

src/locale.cpp

@@ -36,6 +36,7 @@
 #endif
 #include <stdlib.h>
 #include <stdio.h>
+#include "__undef_macros"
 
 // On Linux, wint_t and wchar_t have different signed-ness, and this causes
 // lots of noise in the build log, but no bugs that I know of.