[OpenMP][CMake] Use in-project clang as CUDA->IR compiler.

If available, use the clang that is already built in the same project as
CUDA compiler unless another executable is explicitly defined. This also
ensures the generated deviceRTL IR will be consistent with the version
of Clang.

This patch is required to reliably test OpenMP offloading in a buildbot
without either a two-stage build (e.g. with LLVM_ENABLE_RUNTIMES) or a
separately installed clang on the worker that will eventually become
outdated.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D101265