CppEditor: Clean up CanonicalSymbol

* Remove the static function since it's not really needed
* Make it independent of SemanticInfo

Change-Id: I8d8ea5578f5f9364a26c543277e56bdb5b947049
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
CppEditor: Extract CanonicalSymbol

Change-Id: I42c55f844c4228725251df8c985bfc275925f2ff
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>