[SR-3947] Function Signature Lowering: Investigate splitting passing values with partially opaque layout

[ematejska]

Previous ID	SR-3947
Radar	rdar://problem/31412993
Original Reporter	@ematejska
Type	Bug

Additional Detail from JIRA

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

md5: 59c6f88af346880c7485607a022015b7

Issue Description:

Ahead of ABI Stability and as part of function signature lowering, investigate whether it makes sense to split values with partially opaque layout by passing the non-opaque parts in registers.

[bob-wilson]

@swift-ci create

[bob-wilson]

This would be very complicated and is probably not a huge win. It would help slightly when passing arguments of fixed layout structs that contain resilient structs or enums, which is probably an edge case. We do not expect to do this for the stable ABI in Swift 5. We can consider doing it later for non-public functions.