[SmallSet] Add some simple unit tests.

Reviewers: craig.topper, dblaikie

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D47940


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@334321 91177308-0d34-0410-b5e6-96231b3b80d8