TableGen: Resolve all template args simultaneously in ResolveMulticlassDefARgs

Summary:
Use the new resolver interface more explicitly, and avoid traversing
all the initializers multiple times.

Change-Id: I679e86988b309d19f25e6cca8b0b14ea150198a6

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D43654

llvm-svn: 326708