This change allows setting LIBOMPTARGET_LLVM_LIT_EXECUTABLE and
LIBOMPTARGET_FILECHECK_EXECUTABLE as full path. It also honors
OPENMP_LLVM_TOOLS_DIR which is meant as a common configuration
for both libomp and libomptarget.
Maybe this should be done in a common CMake module, but I'm no expert here.
Differential Revision: https://reviews.llvm.org/D29172
llvm-svn: 294284