[llvm] [cmake] Fix finding modern ounit2

Apparently modern versions of ounit2 can only be found as "ounit2"
rather than "oUnit" version 2.  Update the CMake check to support both
variants.  This makes the OCaml tests run again with ounit2-2.2.4.

Differential Revision: https://reviews.llvm.org/D119079

(cherry picked from commit 919dba92)