Swift: Add flag to omit deprecated enum cases

[kevinmbeaulieu]

Implement feature request described in apollographql/apollo-ios#840

TODO:

• Update CHANGELOG.md* with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

[apollo-cla]

@kevinmbeaulieu: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[designatednerd]

Ignore the unrelated failures in the windows 10 one.

Can you add some tests for this please?

[kevinmbeaulieu]

@designatednerd Added tests for both the enabled & disabled states of the new flag

[designatednerd]

Should this default to false to preserve existing behavior, or would that be overkill?

[kevinmbeaulieu]

Yeah I intended for this to default to false to preserve existing behavior- is that not how it would behave as currently implemented? Looking at the implementation of flags.boolean I saw "specifying a default of false is the same not specifying a default"

[designatednerd]

Ah ok (not exactly a typescript expert over here). I think that should work fine, then.