TableGen: Allow ? in lists

This makes using !dag more convenient in some cases.

Change-Id: I0a8c35e15ccd1ecec778fd1c8d64eee38d74517c

Differential revision: https://reviews.llvm.org/D44111

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@327493 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed