[clang] [clang][ssaf] Introduce entity abstraction for SSAF (PR #169131)

[Yitzhak Mandelbaum via cfe-commits]

https://github.com/ymand requested changes to this pull request.

I'm really excited to see these patches start to land! I reviewed the header files, focusing on the API. I'll try to get back to implementation and tests soon, but wanted to get these out in the meantime.

https://github.com/llvm/llvm-project/pull/169131