TableGen: Remove the ResolveFirst mechanism

Summary:
It is no longer used.

Change-Id: I1e47267d1975d43ad43acd6347f54e958e3b6c86

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D43757

llvm-svn: 326789