TableGen: Move getNewAnonymousName into RecordKeeper

Summary:
So that we will be able to generate new anonymous names more easily
outside the parser as well.

Change-Id: I28f396a7bdbc3ff0c665d466abbd3d31376e21b4

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D43755

llvm-svn: 326787