Inherit from error_category instead of _do_message.

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

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@210626 91177308-0d34-0410-b5e6-96231b3b80d8
Missed these from the previous commit.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@210629 91177308-0d34-0410-b5e6-96231b3b80d8