Issue Description:

You might expect the following code to work:

func aFunction() -> Int?       { return 5 }
func bFunction() throws -> Int { return 4 }

let value = aFunction() ?? try bFunction() // ERROR: Operator can throw but expression is not marked with a ‘try'
print(value)

The workaround is to place 'try' before the entire expression:

let value = try aFunction() ?? bFunction()

This is awkward, because `aFunction` isn't throwable; `bFunction` is. We ought to be able to place the `try` before `bFunction()` and have that propagate out.

The '??' operator is implemented with a rethrowing autoclosure:

public func ?? <T>(optional: T?, defaultValue: @autoclosure () throws -> T)
        rethrows -> T { ... }

The behaviour would likely make sense for all functions which rethrow from autoclosures.