Pitch: Back-deployment for fixed-layout value types - #3571

Hi all, @tshortli and I found a way to do back-deployment for a limited class of types, specifically, @frozen struct and enum types with a fixed layout. The proposal here is to extend the existing @backDeployed(before:) attribute to cover these types. Then, we can adopt it for a number of core types in the standard library: Span family: Span RawSpan MutableSpan MutableRawSpan OutputSpan OutputRawSpan ByteOrder Ref family Ref (only back-deployable to Swift 6.2-era...