Use the in-project clang, llvm-link and opt if available and unless
CMake cache variables specify to use a different compiler. This applies
D101265 to the new DeviceRTL's CMakeLists.txt which was copied before
D101265 was applied.

Fixes the openmp-offloading-cuda-runtime builder which was failing
since D110006.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D110251