Clang: implement requestFollowSymbol plug-in side

Invoke follow symbol in clang backend
if env variable QTC_CLANG_FOLLOW_SYMBOL is 1.
Does not include backend implementation.

Change-Id: Ia20a677830ebdd7f24800af5c5d6e8b1bf579205
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/cpptools/builtineditordocumentprocessor.h

@@ -52,6 +52,8 @@ public:
     bool isParserRunning() const override;
 
     QFuture<CursorInfo> cursorInfo(const CursorInfoParams &params) override;
+    QFuture<SymbolInfo> requestFollowSymbol(int, int, bool) override
+    { return QFuture<SymbolInfo>(); }
 
 private:
     void onParserFinished(CPlusPlus::Document::Ptr document, CPlusPlus::Snapshot snapshot);