Merge #7359

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>