Modifier arguments should allow ExpressionStatements

This fails to parse:

```
pragma solidity ^0.4.4;

contract A {
function yes() public constant returns(bool) {
return true;
}
}

contract MyContract {
A a;
modifier only(bool b) { _; }
function foo() only(a.yes()) {
}
}
```

If `a.yes()` is replaced with `true` it parses fine.

Start by reproducing the parser failure with the Solidity snippet in the issue, then trace how modifier arguments handle `a.yes()` compared with `true`. Done means the expression-statement argument parses successfully without regressing the existing `true` case.

Written by the indexing model from the issue text.