[Frontend] Factor out setupDependencyTrackerIfNeeded

And call from `CompilerInstance::setup`.

Author: hamishknight

include/swift/Frontend/Frontend.h

@@ -582,6 +582,7 @@ class CompilerInstance {
   bool setUpASTContextIfNeeded();
   void setupStatsReporter();
   void setupDiagnosticVerifierIfNeeded();
+  void setupDependencyTrackerIfNeeded();
   Optional<unsigned> setUpCodeCompletionBuffer();
 
   /// Set up all state in the CompilerInstance to process the given input file.

lib/Frontend/Frontend.cpp

@@ -288,9 +288,25 @@ void CompilerInstance::setupDiagnosticVerifierIfNeeded() {
   }
 }
 
+void CompilerInstance::setupDependencyTrackerIfNeeded() {
+  const auto &Invocation = getInvocation();
+  const auto &opts = Invocation.getFrontendOptions();
+
+  if (opts.InputsAndOutputs.hasDependencyTrackerPath() ||
+      !opts.IndexStorePath.empty() || opts.TrackSystemDeps) {
+    // Note that we're tracking dependencies even when we don't need to write
+    // them directly; in particular, -track-system-dependencies affects how
+    // module interfaces get loaded, and so we need to be consistently tracking
+    // system dependencies throughout the compiler.
+    createDependencyTracker(opts.TrackSystemDeps);
+  }
+}
+
 bool CompilerInstance::setup(const CompilerInvocation &Invok) {
   Invocation = Invok;
 
+  setupDependencyTrackerIfNeeded();
+
   // If initializing the overlay file system fails there's no sense in
   // continuing because the compiler will read the wrong files.
   if (setUpVirtualFileSystemOverlays())

lib/FrontendTool/FrontendTool.cpp

@@ -2202,18 +2202,6 @@ int swift::performFrontend(ArrayRef<const char *> Args,
   const DiagnosticOptions &diagOpts = Invocation.getDiagnosticOptions();
   bool verifierEnabled = diagOpts.VerifyMode != DiagnosticOptions::NoVerify;
 
-  if (Invocation.getFrontendOptions()
-          .InputsAndOutputs.hasDependencyTrackerPath() ||
-      !Invocation.getFrontendOptions().IndexStorePath.empty() ||
-      Invocation.getFrontendOptions().TrackSystemDeps) {
-    // Note that we're tracking dependencies even when we don't need to write
-    // them directly; in particular, -track-system-dependencies affects how
-    // module interfaces get loaded, and so we need to be consistently tracking
-    // system dependencies throughout the compiler.
-    Instance->createDependencyTracker(
-        Invocation.getFrontendOptions().TrackSystemDeps);
-  }
-
   if (Instance->setup(Invocation)) {
     return finishDiagProcessing(1, /*verifierEnabled*/ false);
   }