[SR-3730] Technical specification of the layout algorithms

[ematejska]

Previous ID	SR-3730
Radar	rdar://problem/31411200
Original Reporter	@ematejska
Type	Task

Additional Detail from JIRA

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

md5: da7c0c4eedd5aafaec6dfec6b21671f4

Issue Description:

Provide a technical specification of the layout algorithms that future compilers must adhere to in order to ensure binary compatibility as part of locking down ABI Stability

[bob-wilson]

@swift-ci create

[atrick]

The optimizer makes assumptions about layout equivalence. See canUnsafeCastStruct/Tuple/Enum. Move this logic into a central place where we specify ABI requirements.