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

[Simon Camphausen via llvm-commits]

https://github.com/simon-camp approved this pull request.

LGTM. In the future we might want to make the validation of function types more ergonomic if these come up in other places, for example for the conversion of SSA values with function type, like `func.constant`.

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