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/clangcodemodel/clangeditordocumentprocessor.h

@@ -86,6 +86,9 @@ public:
     void setParserConfig(const CppTools::BaseEditorDocumentParser::Configuration config) override;
 
     QFuture<CppTools::CursorInfo> cursorInfo(const CppTools::CursorInfoParams &params) override;
+    QFuture<CppTools::SymbolInfo> requestFollowSymbol(int line,
+                                                      int column,
+                                                      bool resolveTarget) override;
 
     ClangBackEnd::FileContainer fileContainerWithArguments() const;