[libunwind] [libunwind] [SEH] Implement parsing of aarch64 pdata/xdata (PR #137949)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

The parsing itself looks fine.  I don't have any context for how this code is supposed to work, though.

https://github.com/llvm/llvm-project/pull/137949