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

[Tristan Ross via cfe-commits]

RossComputerGuy wrote:

> IMO libclc should not support the standalone build, and this should be version locked to the exact compiler commit.

So it should be built along with the core of LLVM? Also, we package LLVM per version per subproject.

> could this be avoided by changing the Nixpkgs build to install symlinks to all needed tools in a single directory (possibly a temporary directory populated during the libclc build), and making that directory available as `LLVM_TOOLS_BINARY_DIR`?

Maybe, although that sounds a bit hacky.

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