TableGen: Allow lists to be concatenated through '#'

TableGen:  Allow lists to be concatenated through '#'

Currently one can concatenate strings using hash(#),
but not lists, although that would be a natural thing to do.

This patch allows one to write something like:
def : A<!listconcat([1,2], [3,4])>;
simply as :
def : A<[1,2] # [3,4]>;

This was missing feature was highlighted by Nicolai
at FOSDEM talk.

Reviewed by: nhaehnle, hfinkel

Differential Revision: https://reviews.llvm.org/D58895

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@355414 91177308-0d34-0410-b5e6-96231b3b80d8