cmake: Don't install plugins used for examples or tests

Summary:
This patch drops install targets for LLVMHello.so,
TestPlugin.so, and BugpointPasses.so.

Reviewers: chandlerc, beanz, thakis, philip.pfaffe

Reviewed By: chandlerc

Subscribers: SquallATF, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D55965

llvm-svn: 351087