[NFC] Differential Incremental External Dependencies in DependencyTracker

Also perform the plumbing necessary to convince the rest of the compiler that they're just ordinary external dependencies. In particular, we will still emit these depenencies into .d files, and code completion will still index them.

lib/AST/ModuleLoader.cpp

@@ -50,11 +50,22 @@ DependencyTracker::addDependency(StringRef File, bool IsSystem) {
                                      /*IsMissing=*/false);
 }
 
+void DependencyTracker::addIncrementalDependency(StringRef File) {
+  if (incrementalDepsUniquer.insert(File).second) {
+    incrementalDeps.emplace_back(File.str());
+  }
+}
+
 ArrayRef<std::string>
 DependencyTracker::getDependencies() const {
   return clangCollector->getDependencies();
 }
 
+ArrayRef<std::string>
+DependencyTracker::getIncrementalDependencies() const {
+  return incrementalDeps;
+}
+
 std::shared_ptr<clang::DependencyCollector>
 DependencyTracker::getClangCollector() {
   return clangCollector;