[libclc] [libclc] use default paths with find_program when possible (PR #105969)

[Harald van Dijk via cfe-commits]

hvdijk wrote:

Apologies, but I'm having a bit of trouble understanding the scenario that this PR addresses. It looks like it's meant to handle the case where `LLVM_TOOLS_BINARY_DIR` does not contain the LLVM binaries, is that right? In that case, why can `LLVM_TOOLS_BINARY_DIR` not instead be set to a path that does contain the LLVM binaries?

https://github.com/llvm/llvm-project/pull/105969