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

[Marius Brehler via llvm-commits]

marbre wrote:

All review comments are addressed now. Furthermore, I extended the verifier to disallow empty function bodies. @simon-camp it would be awesome if you could scan over one last time.

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