It appears that the standard accidentally removed the default constructor for error_category.  I'm putting it back in.  This fixes http://llvm.org/bugs/show_bug.cgi?id=12321.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@153194 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/system_error b/include/system_error
index 805c646..e9f2b55 100644
--- a/include/system_error
+++ b/include/system_error
@@ -366,8 +366,8 @@
 public:
     virtual ~error_category() _NOEXCEPT;
 
-private:
     error_category() _NOEXCEPT;
+private:
     error_category(const error_category&);// = delete;
     error_category& operator=(const error_category&);// = delete;