[SR-3731] Technical specification for the fixed part of the metadata layout of all language constructs

[ec8ba57e-8db4-4a5b-97d9-d3e186c86593]

Previous ID	SR-3731
Radar	rdar://problem/31411202
Original Reporter	@ematejska
Type	Task

Additional Detail from JIRA

| | | |------------------|-----------------| |Votes | 0 | |Component/s | | |Labels | Task, AffectsABI | |Assignee | None | |Priority | Medium | md5: 708d189dd98d28c0ce183fea78f6d721

Issue Description:

As part of the ABI stability work, provide a technical specification for the fixed part of the metadata layout of all language constructs so that future compilers and tools can continue to read and write them.

[bob-wilson]

@swift-ci create