TableGen: Move GenStrConcat to a helper function in BinOpInit

Summary:
Make it accessible for more users.

Change-Id: Ib05f09ba14e7942ced5d2f24b205efa285e40cd5

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D44196

llvm-svn: 327845