[libclc] [libclc] Update build instructions in readme (PR #111369)

[Fraser Cormack via cfe-commits]

frasercrmck wrote:

> Also do you know if libclc has any tests worth running in CI? It doesn't have a check-libclc so I'm not sure if that's because there's not much to run, or running the tests requires a GPU or something else.
> 
> Context is I've been looking at the pipeline we use for PRs and it doesn't know what to do with libclc so it uses check-all, ideally it would have a specific target because check-all can lead to us running more tests than we need to.

No, there aren't any tests. Well I think there are some `CTest` tests but I seriously doubt those work. I think even `check-all` might not build `libclc`? I might be wrong there. We have added tests downstream but they aren't ready for upstreaming.

I did start adding some tests in #87989 but didn't get very far before I switched focus. Part of the problem is that it can be built out of tree, or with an unknown version of clang, and it's hard to know what other downstreams are doing with it.

https://github.com/llvm/llvm-project/pull/111369