Make _predatesConcurrency ABI/API stable to add

Using _predatesConcurrency is what prevents adding `@Sendable` to things
from breaking the ABI. Yes, technically adding it will break anything
that had `@Sendable` before and is now pre-dating concurrency. After
discussing with Doug, this was the direction we chose to take.

Author: etcwilde

include/swift/AST/Attr.def

@@ -691,7 +691,7 @@ SIMPLE_DECL_ATTR(_noAllocation, NoAllocation,
 SIMPLE_DECL_ATTR(_predatesConcurrency, PredatesConcurrency,
   OnFunc | OnConstructor | OnProtocol | OnGenericType | OnVar | OnSubscript |
   OnEnumElement | UserInaccessible |
-  ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIBreakingToRemove,
   125)
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(_const, CompileTimeConst,