Add nil-collapsing call expression

[rudkx (Mark Lacey)]

If throw becomes an expression of type Never and Never becomes a true bottom type, you'd be able to write this as:

   try? quixify(foo: acquireFoo() ?? throw CannotAcquireFoo, 
                bar: acquireBar() ?? throw CannotAcquireBar,
                baz: acquireBaz())

Of course you could also use plain try and handle the errors explicitly as well.