[Dependency Scanning] Disable validation of Swift dependency modules' existing pre-built candidate binary module files in the scanner, on a non-caching build.

As-is, this default interferes with the incremental build machinery which conservatively assumes that binary module dependencies must cause dependents to be re-built.

lib/Serialization/SerializedModuleLoader.cpp

@@ -455,7 +455,7 @@ SerializedModuleLoaderBase::scanModuleFile(Twine modulePath, bool isFramework,
       isRequiredOSSAModules(), Ctx.LangOpts.SDKName,
       Ctx.SearchPathOpts.DeserializedPathRecoverer, loadedModuleFile);
 
-  if (!Ctx.SearchPathOpts.NoScannerModuleValidation) {
+  if (Ctx.SearchPathOpts.ScannerModuleValidation) {
     // If failed to load, just ignore and return do not found.
     if (loadInfo.status != serialization::Status::Valid) {
       if (Ctx.LangOpts.EnableModuleLoadingRemarks)