[compiler-rt] [libcxx] [libcxxabi] [libunwind] [runtimes][PAC] Harden unwinding when possible (PR #143230)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

Remaining formatting changes differ significantly from the surrounding code. I don't want to intermingle significant changes with formatting changes.

https://github.com/llvm/llvm-project/pull/143230