[libclc] [libclc] Allow default path when looking for llvm-spirv (PR #126071)

[Fraser Cormack via cfe-commits]

https://github.com/frasercrmck commented:

I get where this is coming from, thanks.

However, note that with this change, the priority is changed so that `llvm-spirv` will be picked up from the PATH environment variable *before* `LLVM_TOOLS_BINARY_DIR`.

See the documentation for [find_program](https://cmake.org/cmake/help/latest/command/find_program.html), when `NO_DEFAULT_PATH` is provided:

* step 5: Search the standard system environment variables
* step 7: Search the paths specified by the PATHS option (...)

I'm not too concerned by the other steps as they're cmake-specific options or variables.

However, this does appear to present a change to users (would re-configuring change the location of the llvm-spirv binary in such scenarios?).

Should `LLVM_TOOLS_BINARY_DIR` be made a `HINT` so that it gets searched in step 4 before system paths?

https://github.com/llvm/llvm-project/pull/126071