[mlir] [llvm] [mlir][EmitC] Add func, call and return operations and conversions (PR #79612)

[Marius Brehler via llvm-commits]

marbre wrote:

> @marbre thank you so much for working on this 🙌
> 
> I just gave it a try and it's working great!

Thanks for testing!

> One minor suggestion: it would be nice to expose `populateFuncToEmitcPatterns` to combine it with other conversions without having to run a pass separately.

Sure! I already had this on my list, but unfortunately I didn't get to it on Friday before I had to sign off. Just split the former `FuncToEmitC.{cpp,h}` into `FuncToEmitC.{cpp,h}` and `FuncToEmitCPass.{cpp,h}`. Let me know if you have further requests or suggestions.

https://github.com/llvm/llvm-project/pull/79612