Allow returns values to be ignored in try catch

[seanyoung]

A try catch statement does not need to decode the return values if it is not interested.

interface I {
      function func()   externalpublic returns (int, bool);
}

contract C {
     function test(I i) public {
           try I.func() {
           }
           catch (bytes memory bs) { 
           }
     } 
}

Here func() returns two values but the try statement does not use them at all. This means they do not have abi decoded either.

[xermicus]

I think this change is correct. A codegen test, ultimately proofing that the decoding is no longer happening, would be the cherry on top.

[seanyoung]

I think this change is correct. A codegen test, ultimately proofing that the decoding is no longer happening, would be the cherry on top.

I've added a test