add an experimental feature DeferSendableChecking to defer the sendable checking of some sites. For now, only diagnostics corresponding to non-sendable arguments passed to calls with unsatisfied isolation are deferred. A SIL pass SendNonSendable is added to emit the deferred diagnostics, and ApplyExpr is appropriately enriched to make that deferral possible.

lib/SILOptimizer/PassManager/PassPipeline.cpp

@@ -132,6 +132,7 @@ static void addMandatoryDiagnosticOptPipeline(SILPassPipelinePlan &P) {
   P.addAddressLowering();
 
   P.addFlowIsolation();
+  P.addSendNonSendable();
 
   // Automatic differentiation: canonicalize all differentiability witnesses
   // and `differentiable_function` instructions.