TableGen: Avoid using resolveListElementReference in TGParser

A subsequent change intends to remove resolveListElementReference
entirely. This part of the removal can be split out for better
bisectability.

Change-Id: Ibd762d88fd2d1e2cc116a259e2a27a5e9f9a8b10

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D43561

Change-Id: Ifb695041cef1964ad8a3102f448249501a9243f0
llvm-svn: 325886