[compiler-rt] [rtsan][NFC] Refactor where we unwind the stack (PR #111443)

[Chris Apple via llvm-commits]

cjappl wrote:

Let me know if you have any post-commit reviews @davidtrevelyan - felt comfortable submitting this because you reviewed a preliminary version of this.

Thanks as always for the thorough and quick reviews @fmayer 

https://github.com/llvm/llvm-project/pull/111443