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.

BUG: 444055

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