Import Foundation in codegen

[dfed]

Hi folks! The Swift code-gen today requires that an Objective-C umbrella header be present that has an import <UIKit/UIKit.h> to compile. You can see this for yourself by running code-gen on any request/response that has a Date as a value or parameter. Date comes from Foundation, but there is no import of Foundation. Instead, the generated code implicitly relies on the UIKit import from an umbrella header to pull in Foundation.

This PR explicitly imports Foundation in the generated code, which allows consumers of Swift Apollo code-gen to no longer need an umbrella header.

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

*Make sure changelog entries note which project(s) has been affected. See older entries for examples on what this looks like.

[apollo-cla]

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

[trevor-scheer]

@designatednerd, would you mind taking a look at this when you get the chance? Thanks!

[fassko]

Would be great to get this forward. Thanks!

[designatednerd]

@dfed Do you have a bit of time to rebase this, or do you mind if I take over this PR?

[dfed]

Happy to have you take this PR over the finish line @designatednerd! I’m out on vacation this week.

[designatednerd]

fwiw you don't need to put the link here

[designatednerd]

OK cool - ping me when it's ready for another look

[designatednerd]

🤦‍♀ Completely misread the earlier comment! Will take this over.

[designatednerd]

Works nicely in the test project - let's do this!