Define layout algorithm for structs

The Swift ABI Dashboard (https://swift.org/abi-stability) currently lists 4 separate issues as part of defining the layout algorithm for structs. I am filing this "umbrella" task to summarize the data layout issues for structs so that we can list only one issue on the dashboard.

Here are the 4 tasks that we have been tracking:

https://bugs.swift.org/browse/SR-3723:  Determine if the layout algorithm for structs should layout fields in a different order than declared

https://bugs.swift.org/browse/SR-3724:  Evaluate a fully declaration-order-agnostic layout algorithm for structs

https://bugs.swift.org/browse/SR-3725:  Determine if struct data members are addressable or bit-packed

https://bugs.swift.org/browse/SR-3912:  Decide the cap of alignment for struct layout