[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

llvm-svn: 304357
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: a016efb1dcda61e606e527b73791fbacf1bc9c86
diff --git a/src/strstream.cpp b/src/strstream.cpp
index be94f9c..01523cf 100644
--- a/src/strstream.cpp
+++ b/src/strstream.cpp
@@ -13,6 +13,7 @@
 #include "cstring"
 #include "cstdlib"
 #include "__debug"
+#include "__undef_macros"
 
 _LIBCPP_BEGIN_NAMESPACE_STD