[clang] [PAC] Implement function pointer re-signing (PR #98847)

[Daniil Kovalev via cfe-commits]

https://github.com/kovdan01 commented:

Mostly LGTM. There are several new pretty minor comments, but they can be addressed in a follor-up PR.

The thing which is better to be addressed as a part of this PR is deleting three unneeded member functions from `CodeGenFunction`. IMHO there is no reason to ship this changes right now.

https://github.com/llvm/llvm-project/pull/98847