simplify a switch statement

[glessard]

This change has no effect on functionality, documentation or testing.

The switch in ArgumentSet.flagUpdate (originally in one of the Flag constructors) switches on 3 items, one of which (the return value of values.element(forKey:)) is needed in only one of the 12 possibilities. This moves the call to values.element(forKey:) into the one case where it is needed. The function becomes slightly more efficient, and the switch is more readable (down to 6 possibilities), and no longer needs a default case. Note that the probably-impossible failure mode where values.element(forKey: key returns nil and hasUpdated is true still results in the same behaviour, namely the argument is unused and the function returns true; this eventually leads to a call to ErrorMessageGenerator.unknownOptionMessage.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[glessard]

Superseded by #71