[llvm] [BOLT][binary-analysis] Add initial pac-ret gadget scanner (PR #122304)

[Anatoly Trosinenko via llvm-commits]

https://github.com/atrosinenko commented:

Thank you @kbeyls ! I added a few more comments covering all the files except `NonPacProtectedRetAnalysis.cpp` - in other respects these files look good to me in their current state.

https://github.com/llvm/llvm-project/pull/122304