[SR-5926] Remove Builtin.UnknownObject from the AST and SIL layers

[belkadan]

Previous ID	SR-5926
Radar	None
Original Reporter	@belkadan
Type	Improvement
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Improvement
Assignee	None
Priority	Medium

md5: 36242154e2d697735e7333fa8f4ea47c

Issue Description:

Builtin.UnknownObject represents an opaque pointer value that uses objc_retain for its refcounting. It's pretty much equivalent to AnyObject at this point, so the only reason to keep it around is to have easy access to a prebuilt value witness table for the type. That can be handled entirely by IRGen.

[belkadan]

#11984 tracks removing the last use of Builtin.UnknownObject from the stdlib.

[typesanitizer]

For future reference, there's a PR which has a bunch of implementation: #12325

[belkadan]

Oh, I finished this in #27378 !