Inherit from error_category instead of _do_message.
Inherit from error_category instead of _do_message.

There is no std::_do_message and these classes override message anyway.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@210626 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld