TableGen: Resolve all template args simultaneously in AddSubMultiClass

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

Change-Id: Ia4dcc6d42dd8b65e6079d318c6a202f36f320fee

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D43653

llvm-svn: 326707