Fix PluginsTests failure on Windows buildbots by enabling it everywhere

lit is picking up a stale executable in the unittests tree, which is
failing on Windows.

To simplify the CMake and avoid problems like this in the future, now we
always compile the test, but the test exits successfully when plugins
are not enabled.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@330867 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed