PR25118: move system_header pragma before uses of include_next to avoid extension warnings for people finding libc++ headers via -I paths.

llvm-svn: 249788
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 2999ea0f041659f6ec0cee0fa6ab119c382bfac2
diff --git a/include/complex.h b/include/complex.h
index 7003d31..c235966 100644
--- a/include/complex.h
+++ b/include/complex.h
@@ -18,6 +18,12 @@
 
 */
 
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
 #ifdef __cplusplus
 
 #include <ccomplex>
@@ -28,8 +34,4 @@
 
 #endif  // __cplusplus
 
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#pragma GCC system_header
-#endif
-
 #endif  // _LIBCPP_COMPLEX_H