[llvm] Reland "[BasicBlockUtils] Handle funclets when detaching EH pad blocks" (PR #158435)

[Gábor Spaits via llvm-commits]

spaits wrote:

Thank you very much @rnk for reviewing my PR. I will probably merge it today. 

I have added a few NFC commits. Just cleaning up and more testing.
- Added another test case.
- Added some more comments, when deleting the uses of catchpad/cleanuppad.
- Cleaned up the code a bit:
    - Merged two static functions, that are always called after each other (`zapAllInstructionInDeadBasicBlock` + `deleteBasicBlockFromSuccessor` -> `emptyAndDetachBlock`)
    - Deleted the static function, that was only called from one place after no more explicit `catchswitch` handling (`replaceFuncletPadsRetWithUnreachable`).

https://github.com/llvm/llvm-project/pull/158435