Lock the DUChain when retrieving the declaration for a StructureType
The call to KDevelop::IdentifiedType::declaration requires the DUChain
to be at least read locked, since it calls KDevelop::DUChain::chainForIndex,
which is documented to require a read lock on the DUChain
The commit also removes a (slightly) related misdocumentation of
Helper::resolveAliasDeclaration, which claimed to need a read
lock while doing the locking itself.