[SR-3732] Lock down the layout of a protocol witness table

[ematejska]

Previous ID	SR-3732
Radar	rdar://problem/31411201
Original Reporter	@ematejska
Type	Task
Status	Resolved
Resolution	Done

Additional Detail from JIRA

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

md5: aaddd601f93b5cf0ce1a5be73a9d2bb1

Issue Description:

Protocol witness tables may be created dynamically by the runtime or statically by the compiler. The layout of a protocol witness table is ABI and we need to determine a layout algorithm that also accommodates library evolution concerns, where additional protocol requirements may be added with default fall-backs.

[bob-wilson]

@swift-ci create

[atrick]

To support specialized witness tables in the future, we may need to install a unique identifier in the first word of the witness table to identify the conformance.

[slavapestov]

The unique identifier is done now that @DougGregor added conformance records. If anyone thinks of anything else, please file a new bug.