Add a regression test for SR-9624

[jrose-apple]

It got fixed somewhere along the way in Swift 5.

https://bugs.swift.org/browse/SR-9624

[jrose-apple]

@swift-ci Please smoke test

[slavapestov]

To give credit where credit is due this was fixed by @rjmccall as part of coroutines:

commit c491f1b72152f48390d560f99b6aa0ea243fe19e
Author: John McCall <[email protected]>
Date:   Wed Aug 22 01:46:55 2018 -0400

    Implement reabstraction of inouts in witness and reabstraction thunks.
    
    This is a longstanding gap in the implementation that quite possibly
    never bit anyone in the wild.  I've only gotten around to implementing
    it now because this same code path will be used to reabstract inout
    yields.  That means that, rather than only affecting functions with
    an abstraction difference in an inout parameter type, this can affect
    all storage with any abstraction difference in the stored value type.
    So it's time to fill in this gap.