[clang] [CIR] Add is_implicit to ReturnOp (PR #174832)

[Henrich Lauko via cfe-commits]

================
@@ -714,7 +714,7 @@ def CIR_ReturnOp : CIR_Op<"return", [
 
   // The return operation takes an optional input operand to return. This
   // value must match the return type of the enclosing function.
-  let arguments = (ins Variadic<CIR_AnyType>:$input);
+  let arguments = (ins Variadic<CIR_AnyType>:$input, UnitAttr:$is_implicit);
 
   // The return operation only emits the input in the format if it is present.
   let assemblyFormat = "($input^ `:` type($input))? attr-dict ";
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xlauko wrote:

```suggestion
  let assemblyFormat = [{
    ($input^ `:` type($input))? (`implicit` $is_implicit^)? attr-dict 
  }];
```

https://github.com/llvm/llvm-project/pull/174832