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

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

> Would backporting this be worthwhile?

I guess it could be considered. However in practice I'm not aware of any external cases that actually use the "force unwinding" functionality, outside of the libunwind/libcxxabi testsuite. My main motivation is having the `check-unwind` and `check-cxxabi` testsuites passing in Windows on ARM configurations. It would of course be nice to have that on the 20.x release branch too, but that also requires another backport in https://github.com/llvm/llvm-project/pull/136449 which isn't clear whether it can be backported or not.

Then again, this is straightforward and simple to backport, so I guess we could. (But we'd want #137950 and #137951 to go with it then - I'll merge them soon too.)

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