Implement Attr dumping in terms of visitors

Remove now-vestigial dumpType and dumpBareDeclRef methods. The old
tablegen generated code used to expect them to be present, but the new
generated code has no such requirement.

Reviewers: aaron.ballman

Subscribers: mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D55492

llvm-svn: 350958