[SR-7152] Investigate a Better Collection for CaseIterable Synthesis

[CodaFi]

Previous ID	SR-7152
Radar	None
Original Reporter	@CodaFi
Type	Improvement

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Improvement, StarterBug
Assignee	None
Priority	Medium

md5: a133635420a390cef5a90e74eb062506

Issue Description:

As of this issue, the derived collection is an Array. We should investigate more effective collections before the final branch date for Swift 4.2/5

[sjavora]

Hey @CodaFi, is this still relevant? The deadline of Swift 4.2/5 makes me think this has to do with ABI stability, but I might easily be wrong.