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/AST/ASTPrinter.cpp

@@ -3465,6 +3465,10 @@ static bool usesFeatureParameterPacks(Decl *decl) {
   return false;
 }
 
+static bool usesFeatureDeferredSendableChecking(Decl *decl) {
+  return false;
+}
+
 /// Suppress the printing of a particular feature.
 static void suppressingFeature(PrintOptions &options, Feature feature,
                                llvm::function_ref<void()> action) {