[clang] [CIR] Function type return type improvements (PR #128787)

[Henrich Lauko via cfe-commits]

xlauko wrote:

You don't need to write custom printer/parser for return type. As suggested here https://github.com/llvm/clangir/pull/1413
These can be autogenerated from assembly format:

```
let assemblyFormat = [{
    `<` custom<FuncTypeParams>($inputs, $varArg)  (`->` $optionalReturnType^)? `>`
  }];
```

https://github.com/llvm/llvm-project/pull/128787