[Frontend] Load standard libarary in CompilerInstance::setup

Instead of checking that the stdlib can be loaded in a variety of places, check it when setting up the compiler instance. This required a couple more checks to avoid loading the stdlib in cases where it’s not needed.

To be able to differentiate stdlib loading failures from other setup errors, make `CompilerInstance::setup` return an error message on failure via an inout parameter. Consume that error on the call side, replacing a previous, more generic error message, adding error handling where appropriate or ignoring the error message, depending on the context.

include/swift/Frontend/ModuleInterfaceLoader.h

@@ -313,6 +313,9 @@ struct ModuleInterfaceLoaderOptions {
     case FrontendOptions::ActionType::TypecheckModuleFromInterface:
       requestedAction = FrontendOptions::ActionType::Typecheck;
       break;
+    case FrontendOptions::ActionType::ScanDependencies:
+      requestedAction = Opts.RequestedAction;
+      break;
     default:
       requestedAction = FrontendOptions::ActionType::EmitModuleOnly;
       break;