Fix swift build warning - 'default will never be executed'

[therealbnut]

There is a build warning when running utils/build-script:

build/Ninja-DebugAssert/swift-macosx-x86_64/stdlib/private/StdlibUnittest/8/StdlibUnittest.swift:989:5: warning: default will never be executed
    default:
    ^

The default case is necessary, but produces a warning in non-resilient builds. This PR keeps the default case, but temporarily removes another (equivalent) case to ensure the switch is:

• exhaustive
• not redundant

[hughbe]

@swift-ci please smoke test

[therealbnut]

Thanks @hughbe

[therealbnut]

@airspeedswift I noticed you added the FIXME near this code, are you the right person to ping for review?

[airspeedswift]

@therealbnut thanks, that's a good workaround.

@slavapestov do you know what the deal is with the real fix? Just wondering if we'll lose track without the warning still firing...

[airspeedswift]

@swift-ci Please test

[airspeedswift]

want to run a full test rather than just smoke since this is in the unit testing framework...

[therealbnut]

@airspeedswift I just found this issue SR-3278, which seems to refer to this code. SR-3278 can probably be closed if this PR is merged.

Perhaps at minimum a new issue should be added for the "real fix"? I had a look but I couldn't find one.

[therealbnut]

Thanks for your review :)

[therealbnut]

@slavapestov I believe this PR is just waiting for your feedback on @airspeedswift's question, just a reminder, thanks :)

[slavapestov]

@airspeedswift The real fix is to not emit the warning about the redundant default statement if the enum is not explicitly marked fixed layout, even in non-resilient builds. However that is a language change, so at the very least it can't happen until we're ready to flip the switch to building the stdlib and related code in Swift 4 mode.

[airspeedswift]

OK thanks. Will try and remember to put this back when we have that.

[therealbnut]

thanks @airspeedswift for reviewing and @slavapestov for your help :)