`CLANG_ENABLE_CIR` support in standalone builds

[mgorny]

ClangIR currently hard-blocks CLANG_BUILT_STANDALONE. Is there a technical reason why it can't work, or would it be fine if I tried making a pull request to make these work? Particularly, can ClangIR work against libMLIR dylib?

[thesamesam]

cc @llvm/pr-subscribers-clangir

[lanza]

Hey @mgorny, I don't think we're aware of any reason why it wouldn't work. Just none of us that work on it have ever tried. If you would like to try it out I'd suggest doing it on the incubator project at https://github.com/llvm/clangir since what's currently in tree here is very minimal and surely won't expose all problems you'd actually run in to.

[bcardosolopes]

I second @lanza, once we land there we can also land it here to unblock standalone builds in upstream

[mgorny]

Okay, I'll probably try achieving something next weekend. That said, I guess I'll have to specifically match system LLVM to the version in ClangIR first.

[mgorny]

I'm sorry but due to unforeseen circumstances I was out most of the day, and I won't manage it this weekend.