C++ headers should be more aggressively wrapped in extern "C++".

This is kind of a mess. Some projects will wrap our public headers in
extern "C", so we use extern "C++" around our C++ APIs. However this
needs to be done when including C++ standard library headers too since
they don't always, themselves, guard against being wrapped in extern
"C".

Change-Id: Ib7dd4a6f69ca81dd525ecaa1418b3b7ba85b6579
Reviewed-on: https://boringssl-review.googlesource.com/18504
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/span.h

@@ -19,13 +19,11 @@
 
 #if !defined(BORINGSSL_NO_CXX)
 
+extern "C++" {
+
 #include <algorithm>
 #include <type_traits>
 
-#include "../crypto/internal.h"
-
-extern "C++" {
-
 namespace bssl {
 
 template <typename T>