Support: Re-implement dwarf::TagString() using a .def file, NFC

Also re-implements the `dwarf::Tag` enumerator.  I've moved the mock
tags into the enumerator since there's no other way to do this.  Really
they shouldn't be used at all (they're just a hack to identify
`MDNode`s, but we have a class hierarchy for that now).

llvm-svn: 228030