Swift Codegen: Split up appending of fragments to query document

[andybest]

Splits the appending of fragments to the query document into separate
sub expressions to fix an issue where a query that references lots of fragments
causes the Swift type checker to fail with the following message:

error: the compiler is unable to type-check this expression in reasonable time; try breaking up the expression into distinct sub-expressions

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]

@andybest: 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/

[martijnwalraven]

Thanks for the fix, this looks good to me. I wish we had tests that referenced more than one fragment, but since we're replacing codegen it may not be worth investing in that.

[designatednerd]

Apologies for the delay - I just tested the artifact from this against iOS. It does make some small changes in terms of how we concatenate fragment definitions, but I believe those should have a negligible impact on performance. I'll wait to merge this until Monday for "Seems like a bad weekend to deploy" reasons and then I'll cut a new version, add the new version to the iOS SDK, and cut a new version of that. Whee!