[AutoDiff] Update @differentiable attribute API/ABI status.

Mark `@differentiable` attribute as:
- ABI breaking to add/remove.
- API breaking to remove.
- API stable to add.

Author: dan-zheng

include/swift/AST/Attr.def

@@ -506,7 +506,7 @@ DECL_ATTR(_projectedValueProperty, ProjectedValueProperty,
 DECL_ATTR(differentiable, Differentiable,
   OnAccessor | OnConstructor | OnFunc | OnVar | OnSubscript | LongAttribute |
   AllowMultipleAttributes |
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIStableToRemove,
   90)
 DECL_ATTR(differentiating, Differentiating,
   OnFunc | LongAttribute | AllowMultipleAttributes |