[DependencyScanner] Change the scanner order to resolve placeholders first.

This is meant to address a problem that arises on incremental package builds:

A re-scan on an already-built package instead of a placeholder dependency produces a graph that contains a dependency consisting solely of the previously-built swiftmodule.
This is not the behaviour we would like. Instead, we should respect that this is a placeholder dependency and ensure that the dependency graph for that dependency itself captures the fact that a previously-built module exists using compiledModuleCandidates field of the dependency graph.

lib/Serialization/ModuleDependencyScanner.cpp

@@ -176,13 +176,19 @@ Optional<ModuleDependencies> SerializedModuleLoaderBase::getModuleDependencies(
   auto moduleId = Ctx.getIdentifier(moduleName);
   // Instantiate dependency scanning "loaders".
   SmallVector<std::unique_ptr<ModuleDependencyScanner>, 2> scanners;
-  scanners.push_back(std::make_unique<ModuleDependencyScanner>(
-      Ctx, LoadMode, moduleId, delegate));
+  // Placeholder dependencies must be resolved first, to prevent the ModuleDependencyScanner
+  // from first discovering artifacts of a previous build. Such artifacts are captured
+  // as compiledModuleCandidates in the dependency graph of the placeholder dependency module
+  // itself.
   scanners.push_back(std::make_unique<PlaceholderSwiftModuleScanner>(
       Ctx, LoadMode, moduleId, Ctx.SearchPathOpts.PlaceholderDependencyModuleMap,
       delegate));
+  scanners.push_back(std::make_unique<ModuleDependencyScanner>(
+      Ctx, LoadMode, moduleId, delegate));
 
   // Check whether there is a module with this name that we can import.
+  assert(isa<PlaceholderSwiftModuleScanner>(scanners[0].get()) &&
+         "Expected PlaceholderSwiftModuleScanner as the first dependency scanner loader.");
   for (auto &scanner : scanners) {
     if (scanner->canImportModule({moduleId, SourceLoc()})) {
       // Record the dependencies.