libcxx: Move #include_next <math.h> out of header guard in wrapper header.

Code on Windows expects to be able to do:

 #define _USE_MATH_DEFINES
 #include <math.h>

and receive the definitions of mathematical constants, even if <math.h>
has previously been included. To support this scenario, re-include
<math.h> every time the wrapper header is included.

Differential Revision: https://reviews.llvm.org/D42403

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@323490 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/depr/depr.c.headers/math_h.sh.cpp

@@ -0,0 +1,23 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// RUN: %compile -fsyntax-only
+
+#ifdef _MSC_VER
+
+#include <math.h>
+
+#define _USE_MATH_DEFINES
+#include <math.h>
+
+#ifndef M_PI
+#error M_PI not defined
+#endif
+
+#endif