[llvm] [IR][NFC] Use `SwitchInst::defaultDestUndefined` (PR #134199)

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

Can you please also rename the method to defaultDestUnreachable while you are here? The current name is unnecessarily misleading.

https://github.com/llvm/llvm-project/pull/134199