[NFCI] [ClangScanDeps] [P1689] Use PreprocessorOnly Action for P1689

It is fine enough to use PreprocessorOnly action for P1689 format. We
don't need to read any PCH or module files.

clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp

@@ -430,7 +430,9 @@ class DependencyScanningAction : public tooling::ToolAction {
 
     std::unique_ptr<FrontendAction> Action;
 
-    if (ModuleName)
+    if (Format == ScanningOutputFormat::P1689)
+      Action = std::make_unique<PreprocessOnlyAction>();
+    else if (ModuleName)
       Action = std::make_unique<GetDependenciesByModuleNameAction>(*ModuleName);
     else
       Action = std::make_unique<ReadPCHAndPreprocessAction>();