Comment OSLogOptimization: add a FIXME to handle OSSA correctly

Author: atrick

lib/SILOptimizer/Mandatory/OSLogOptimization.cpp

@@ -950,6 +950,10 @@ static void replaceAllUsesAndFixLifetimes(SILValue foldedVal,
   // destroy foldedVal at the end of the borrow scope.
   assert(originalVal.getOwnershipKind() == OwnershipKind::Guaranteed);
 
+  // FIXME: getUniqueBorrowScopeIntroducingValue may look though various storage
+  // casts. There's no reason to think that it's valid to replace uses of
+  // originalVal with a new borrow of the the "introducing value". All casts
+  // potentially need to be cloned.
   Optional<BorrowedValue> originalScopeBegin =
       getUniqueBorrowScopeIntroducingValue(originalVal);
   assert(originalScopeBegin &&