[CodeCompletion] Update for SE-0345 shorthand optional binding

Suggest visible optional values after 'if let'.

rdar://90399603

include/swift/Parse/CodeCompletionCallbacks.h

@@ -236,6 +236,8 @@ public:
 
   virtual void completeTypeAttrBeginning() {};
 
+  virtual void completeOptionalBinding(){};
+
   /// Signals that the AST for the all the delayed-parsed code was
   /// constructed.  No \c complete*() callbacks will be done after this.
   virtual void doneParsing() = 0;