TableGen: Introduce an abstract variable resolver interface

Summary:
The intention is to allow us to more easily restructure how resolving is
done, e.g. resolving multiple variables simultaneously, or using the
resolving mechanism to implement !foreach.

Change-Id: I4b976b54a32e240ad4f562f7eb86a4d663a20ea8

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D43564

llvm-svn: 326704