Support: Make error_category's constructor public

Since our error_category is based on the std one, we should have the
same visibility for the constructor.  This also allows us to avoid
using the _do_message implementation detail in our own categories.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203998 91177308-0d34-0410-b5e6-96231b3b80d8