Add ClangIR to MLIR users

[jryans]

I was browsing the MLIR docs recently and encountered the users of MLIR list.

I think it would make sense to add ClangIR to the list, as it seems like an important MLIR-using project to mention. (I would add it myself, but I don't know that I can write an appropriate summary of ClangIR. 😇)

[bcardosolopes]

If you'd still want to add, I suggest summary from the website: "ClangIR is a high-level representation in Clang that reflects aspects of the C/C++ languages and their extensions. It is implemented using MLIR and occupies a position between Clang’s AST and LLVM IR."

[bcardosolopes]

You can tag me or add as a reviewer and I can help out if you need anything