C++: Do not share dependency table when copying a Snapshot

There are multiple situations were the snapshot is copied and
access/modifications to the dependency table data is made in other
threads, e.g.:

    * FindUsages called by ProcessFile
    * findReferences() in cppuseselectionsupdater.cpp
    * CppModelManager::projectPartFromDependencies() (clang path)

Change-Id: Ided1c5350f339c5bc960b87b32c78ccc21fa61f6
Reviewed-by: hjk <hjk121@nokiamail.com>