[incrParse] Support incremental parsing for edited files

include/swift/Parse/Parser.h

@@ -521,6 +521,13 @@ public:
   /// \brief Skip until the next '#else', '#endif' or until eof.
   void skipUntilConditionalBlockClose();
 
+  /// If the parser is generating only a syntax tree, try loading the current
+  /// node from a previously generated syntax tree.
+  /// Returns \c true if the node has been loaded and inserted into the current
+  /// syntax tree. In this case the parser should behave as if the node has
+  /// successfully been created.
+  bool loadCurrentSyntaxNodeFromCache();
+
   /// Parse an #endif.
   bool parseEndIfDirective(SourceLoc &Loc);