7359: ItemTree: store a mapping from blocks to inner items r=jonas-schievink a=jonas-schievink
To do name resolution within block expressions, we need to know which inner items are located inside each block expression. This adds such a mapping to `ItemTree`, replacing the previous one, which was seemingly unused other than to access all the inner items.
This also assigns `AstId`s to block expressions, which is needed to store the mapping in salsa.
Co-authored-by: Jonas Schievink <jonasschievink@gmail.com>
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