[OCaml] [cmake] Use LLVM_LIBRARY_DIR instead of LLVM_LIBRARY_OUTPUT_INTDIR.

The latter variable is internal.

Original patch by Ramkumar Ramachandra <artagnon@gmail.com>
[lit] Make config.llvm_lib_dir available on cmake, too.

The OCaml tests require config.llvm_lib_dir to determine
the OCaml package search path.
[OCaml] Fix bitrot in tests.