TableGen: Resolve all template args simultaneously in AddSubClass

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

Add a test case for a pattern that was broken by an earlier version
of this change.

An additional change is that we now remove *all* template arguments
after resolving them.

Change-Id: I86c828c8cc84c18b052dfe0f64c0d5cbf3c4e13c

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D43652

llvm-svn: 326706