[libSyntax] Allow syntax cache reuse info to be passed back via SourceKit

unittests/SourceKit/SwiftLang/EditingTest.cpp

@@ -100,6 +100,13 @@ private:
   bool handleSerializedSyntaxTree(StringRef Text) override { return false; }
   bool syntaxTreeEnabled() override { return false; }
   bool forceLibSyntaxBasedProcessing() override { return false; }
+
+  bool syntaxReuseInfoEnabled() override { return false; }
+
+  bool handleSyntaxReuseRegions(
+      std::vector<SourceFileRange> ReuseRegions) override {
+    return false;
+  }
 };
 
 struct DocUpdateMutexState {