* Add a parameter "preferProxyContext" to "DUChainUtils::standardContextForUrl".
Only proxy-context know exactly about all the includes, and thus they need to be used when deciding whether the duchain needs to be updated.

* After switching the document and after refactoring, decide whether the duchain needs to be updated based on the proxy context.

In C++, this fixes the problem that often the files weren't updated even though one of their included headers was changed.