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

[Anatoly Trosinenko via llvm-commits]

https://github.com/atrosinenko commented:

Thanks @kbeyls! Here are a few more comments, most of them are trivial nit-picking.

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