[CoroutineAccessors] Control ABI via flag.

include/swift/AST/SILOptions.h

@@ -338,6 +338,10 @@ public:
   // Whether to allow merging traps and cond_fails.
   bool MergeableTraps = false;
 
+  /// Whether the @yield_once_2 convention is used by accessors added with the
+  /// CoroutineAccessors feature (i.e. read2/modify2).
+  bool CoroutineAccessorsUseYieldOnce2 = false;
+
   SILOptions() {}
 
   /// Return a hash code of any components from these options that should