[AutoDiff] Plumb witness derivative generic signatures through SILGen. #40063
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When SILGenWitnessTable creates a decl ref for the witness of a derivative function requirement, it is using the requirement's derivative generic signature in the resulting witness decl ref. This is wrong because the witness may have a different derivative generic signature than the requirement, leading to a crash. This bug was never discovered because GSB's dark magic made it "just work", until requirement machine.
The fix is to store the matched witness derivative generic signature in
Witness
during type checking, and during witness table generation, use the witness' generic signature to create a witness decl ref.Also brings back #39505 by reverting the revert (#40057), now that the blocking issue was fixed.
Resolves rdar://84716758, rdar://84213107 and rdar://84987079.