[clang] [CIR] Better handling of `void` function return (PR #128089)

[Henrich Lauko via cfe-commits]

================
@@ -375,11 +410,48 @@ void printFuncTypeArgs(mlir::AsmPrinter &p, mlir::ArrayRef<mlir::Type> params,
   p << ')';
 }
 
+// Use a custom parser to handle the optional return and argument types without
+// an optional anchor.
+static mlir::ParseResult parseFuncType(mlir::AsmParser &p,
+                                       mlir::Type &optionalReturnTypes,
+                                       llvm::SmallVector<mlir::Type> &params,
+                                       bool &isVarArg) {
+  if (failed(parseFuncTypeReturn(p, optionalReturnTypes)))
+    return failure();
+  return parseFuncTypeArgs(p, params, isVarArg);
+}
+
+static void printFuncType(mlir::AsmPrinter &p, mlir::Type optionalReturnTypes,
+                          mlir::ArrayRef<mlir::Type> params, bool isVarArg) {
+  printFuncTypeReturn(p, optionalReturnTypes);
----------------
xlauko wrote:

```suggestion
  printFuncTypeReturn(p, optionalReturnType);
```

https://github.com/llvm/llvm-project/pull/128089