[Unittests] Fix plugins test

Summary:
Currently the PluginsTests.LoadPlugin unit test is failing in
LLVM configurations that have LLVM_EXPORT_SYMBOLS_FOR_PLUGINS enabled
because the EnableABIBreakingChecks symbol is missing.

This patch fixes the issue by linking some additional libraries to the
test plugin if LLVM_EXPORT_SYMBOLS_FOR_PLUGINS is enabled.

Reviewers: philip.pfaffe

Reviewed By: philip.pfaffe

Subscribers: mgorny, llvm-commits, rogfer01

Differential Revision: https://reviews.llvm.org/D45811

llvm-svn: 330329