[compilter-rt] Add unittests for interception library

Summary:
This patch is adding unittests for the interception library.


Reviewers: rnk

Subscribers: majnemer, llvm-commits, wang0109, chrisha, tberghammer, danalbert, srhines

Differential Revision: http://reviews.llvm.org/D21980

llvm-svn: 274657