[yaml2obj] Move core yaml2obj code into lib and include for use in unit tests

Reviewers: jhenderson, rupprecht, MaskRay, grimar, labath

Reviewed By: rupprecht

Subscribers: seiya, mgorny, sbc100, hiraditya, aheejin, jakehehrlich, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65255

llvm-svn: 368021