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

llvm-svn: 323490

libcxx/include/math.h

@@ -8,6 +8,16 @@
 //
 //===----------------------------------------------------------------------===//
 
+// This include lives outside the header guard in order to support an MSVC
+// extension which allows users to do:
+//
+// #define _USE_MATH_DEFINES
+// #include <math.h>
+//
+// and receive the definitions of mathematical constants, even if <math.h>
+// has previously been included.
+#include_next <math.h>
+
 #ifndef _LIBCPP_MATH_H
 #define _LIBCPP_MATH_H
 
@@ -298,8 +308,6 @@ long double    truncl(long double x);
 #pragma GCC system_header
 #endif
 
-#include_next <math.h>
-
 #ifdef __cplusplus
 
 // We support including .h headers inside 'extern "C"' contexts, so switch

libcxx/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