Dimitry Andric:  many visibility fixes.  Howard:  Much appreciated.  Can you send me a patch to CREDITS.TXT?

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@163862 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/system_error b/include/system_error
index e9f2b55..9f7e4e9 100644
--- a/include/system_error
+++ b/include/system_error
@@ -354,12 +354,12 @@
     : true_type { };
 #endif
 
-class error_condition;
-class error_code;
+class _LIBCPP_VISIBLE error_condition;
+class _LIBCPP_VISIBLE error_code;
 
 // class error_category
 
-class __do_message;
+class _LIBCPP_HIDDEN __do_message;
 
 class _LIBCPP_VISIBLE error_category
 {
@@ -387,7 +387,7 @@
     _LIBCPP_ALWAYS_INLINE
     bool operator< (const error_category& __rhs) const _NOEXCEPT {return this < &__rhs;}
 
-    friend class __do_message;
+    friend class _LIBCPP_HIDDEN __do_message;
 };
 
 class _LIBCPP_HIDDEN __do_message