swift234

Allow [prefix] and [postfix] functions to overload one another.

Previously, the type checker would complain because a prefix function and
a postfix function (correctly) have the same type. However, the fixity
can usually* be used to disambiguate and is mangled into the symbol name,
so this isn't actually a problem.

* References to operators may still be ambiguous, but that's all right:
    var incr : (Int) -> () = (++)

<rdar://problem/12954722>

Swift SVN r7812