CppEditor: refactor FollowSymbol

Create an interface to get the ability to use
another FollowSymbol implementation

Change-Id: I5802f62523ff3ee47b8a14e487adf43edcb6c9b1
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/clangcodemodel/clangcodemodel.qbs

@@ -71,6 +71,8 @@ QtcPlugin {
         "clangfixitoperation.h",
         "clangfixitoperationsextractor.cpp",
         "clangfixitoperationsextractor.h",
+        "clangfollowsymbol.cpp",
+        "clangfollowsymbol.h",
         "clangfunctionhintmodel.cpp",
         "clangfunctionhintmodel.h",
         "clanghighlightingmarksreporter.cpp",