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

[Erich Keane via cfe-commits]

================
@@ -287,32 +287,43 @@ def CIR_BoolType :
 def CIR_FuncType : CIR_Type<"Func", "func"> {
   let summary = "CIR function type";
   let description = [{
-    The `!cir.func` is a function type. It consists of a single return type, a
-    list of parameter types and can optionally be variadic.
+    The `!cir.func` is a function type. It consists of an optional return type,
+    a list of parameter types and can optionally be variadic.
 
     Example:
 
     ```mlir
-    !cir.func<!bool ()>
-    !cir.func<!s32i (!s8i, !s8i)>
-    !cir.func<!s32i (!s32i, ...)>
+    !cir.func<()>
+    !cir.func<() -> bool>
+    !cir.func<(!s8i, !s8i)>
+    !cir.func<(!s8i, !s8i) -> !s32i>
+    !cir.func<(!s32i, ...) -> !s32i>
     ```
   }];
 
   let parameters = (ins ArrayRefParameter<"mlir::Type">:$inputs,
-                        "mlir::Type":$returnType, "bool":$varArg);
+                        "mlir::Type":$optionalReturnType, "bool":$varArg);
+  // Use a custom parser to handle the argument types and optional return
----------------
erichkeane wrote:

```suggestion
  // Use a custom parser to handle the argument types and optional return.
```

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