[clang][ssaf] Add JSON serialization support for `TUSummary::LinkageTable`
This change adds full read/write support for the
`TUSummary::LinkageTable` field that maps each entity to its linkage
kind. The deserialization step validates that the set of entity ids in
`LinkageTable` exactly matches the set in `IdTable` in a single `O(N log
N)` pass. Existing tests have been updated and new tests have been added
to ensure 100% coverage of the new code.