unittests: Use yaml2obj as a library instead of an external process

Summary:
Recently, yaml2obj has been turned into a library. This means we can use
it from our unit tests directly, instead of shelling out to an external
process. This patch does just that.

Reviewers: JDevlieghere, aadsm, espindola, jdoerfert

Subscribers: emaste, mgorny, arichardson, MaskRay, jhenderson, abrachet, lldb-commits

Differential Revision: https://reviews.llvm.org/D65949

llvm-svn: 369374