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

llvm-svn: 355414