[SSAF] Merge summary data for entities in ssaf-linker

This PR introduces summary data aggregation at a single TU scope. For a given entity, each declaration can contribute to the summary data and we aggregate these contribution.
[SSAF] Properly handle contributors with multiple declarations
https://github.com/llvm/llvm-project/pull/204482

We need to implement aggregation also in ssaf-linker to cover the scenario where declarations in multiple different TUs contribute summary data.

We expect that most summaries will extract data from definitions, rather than just declarations but we need to develop this functionality nevertheless.

Start by reading the linked PR 204482 and then trace how ssaf-linker currently aggregates summary data across translation units. Done means declarations from different TUs can contribute to a single entity's summary data, including contributors with multiple declarations; the payload names no specific files or tests.

Written by the indexing model from the issue text.