[SR-6718] Linux Interoperability: Eliminate items in class metadata layout that aren't relevant to non-objc builds

[AnnaZaks]

Previous ID	SR-6718
Radar	rdar://problem/18801263
Original Reporter	@AnnaZaks
Type	Bug
Status	Resolved
Resolution	Done

Additional Detail from JIRA

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

md5: 1801085f2cc5dfb398494cbf214ff76d

Issue Description:

There are various places in the metadata generation where NULL values are being injected simply because there’s some objective-c related field that isn’t important on a non-objc platform. We should remove these and make sure that things still work. (Note that the debugger wants to preserve some sort of magic number that can be used to identify classes. On ObjC platforms the debugger uses the Swift class's ObjC data to do that.)

[belkadan]

Reminded of this again today. I don't think it would be too difficult, so maybe a good task to assign to someone who's already familiar with the compiler a bit and wants to dive into IRGen and the runtime?

As for a "magic number to identify classes", the value witness table for every class is the same (since it's "just" for native-object references), so we can probably use that: $sBoWV

[belkadan]

Apparently @davezarzycki got around to this in #31811 so it'll make it into 5.4 (or 6, whatever's next).

[belkadan]

Tagging @mikeash to make sure the Radar gets closed too.

[mikeash]

Thanks Jordan, you have pleased the Lords of Radar.