Fix for emit_reference_dependencies_without_primary_file warning required after changes to introduce multiple primary files to CompilerInstance.

Revealed by test Frontend/dependencies.swift.

Author: David Ungar

lib/FrontendTool/FrontendTool.cpp

@@ -734,6 +734,9 @@ static bool performCompile(CompilerInstance &Instance,
                                opts);
 
   if (shouldTrackReferences) {
+    if (Instance.getPrimarySourceFiles().empty())
+      Context.Diags.diagnose(
+          SourceLoc(), diag::emit_reference_dependencies_without_primary_file);
     for (auto *SF : Instance.getPrimarySourceFiles()) {
       emitReferenceDependencies(Context.Diags, SF,
                                 *Instance.getDependencyTracker(), opts);

lib/FrontendTool/ReferenceDependencies.cpp

@@ -126,11 +126,7 @@ bool swift::emitReferenceDependencies(DiagnosticEngine &diags,
                                       SourceFile *SF,
                                       DependencyTracker &depTracker,
                                       const FrontendOptions &opts) {
-  if (!SF) {
-    diags.diagnose(SourceLoc(),
-                   diag::emit_reference_dependencies_without_primary_file);
-    return true;
-  }
+  assert(SF && "Cannot emit reference dependencies without a SourceFile");
 
   // Before writing to the dependencies file path, preserve any previous file
   // that may have been there. No error handling -- this is just a nicety, it