[CMake] Link unittests only against libLLVM.so, if available.

We can only link against libLLVM.so or the individual libLLVM*.so
components, but not both of them. Doing so results in these components
exist twice in the programs address space, since it is already contained
in libLLVM.so. The observable effect of this is that command line
switches are registered multiple times (once for each instance),
which is an error.

This fixes llvm.org/PR32735.

Reported-by: Singapuram Sanjay Srivallabh <singapuram.sanjay@gmail.com>
llvm-svn: 301020