[SR-4331] Explore "exploding" existential parameters

[bob-wilson]

Previous ID	SR-4331
Radar	rdar://problem/31408032
Original Reporter	@bob-wilson
Type	Improvement
Status	Resolved
Resolution	Won't Do

Additional Detail from JIRA

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

md5: b99e8ffd9829bb4dfba925679bf97f5f

Issue Description:

From the Data Layout / Existential Containers section of the ABI Stability manifesto:

We should also explore "exploding" existential parameters, i.e. converting an existential parameter into a protocol-constrained generic parameter

[bob-wilson]

@swift-ci create

[bob-wilson]

This would be a huge change and we're running out of time. At this point, we're not planning to attempt this.