[swift] make enums conform to CaseIterable

[4brunu]

It would be nice to make the enums conform to CaseIterable protocol for the Swift compiler to be able to generate a property called allCases that provides an array of all cases in the enum.

This PR fixes #4842

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@jgavris (2017/07) @ehyche (2017/08) @Edubits (2017/09) @jaz-ah (2017/09) @4brunu (2019/11)

[4brunu]

Close in favor of #4086