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>

src/libs/cplusplus/CppDocument.h

@@ -433,7 +433,7 @@ public:
 private:
     void allIncludesForDocument_helper(const QString &fileName, QSet<QString> &result) const;
 
-    mutable QSharedPointer<DependencyTable> m_deps;
+    mutable DependencyTable m_deps;
     Base _documents;
 };