When plugins aren't enabled, don't try to run plugins tests. Don't
enable plugins unconditionally based on the platform, instead check
if LLVM shared library is actually being built which may not be the
case for every host configuration, even if the host itself supports
plugins.

This addresses test failures introduced by r360891/D59464.

Differential Revision: https://reviews.llvm.org/D62050

llvm-svn: 360991