TableGen: Fix type deduction for !foreach

Summary:
In the case of !foreach(id, input-list, transform) where the type of
input-list is list<A> and the type of transform is B, we now correctly
deduce list<B> as the type of the !foreach.

Change-Id: Ia19dd65eecc5991dd648280ba6a15f6a20fd61de

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D43555

llvm-svn: 325797