[llvm] [win] Fix EH Cont Guard targets when SEH personality is used (PR #129612)

[Daniel Paoliello via llvm-commits]

dpaoliello wrote:

> Thanks a lot for the fixing @dpaoliello !
> 
> > In fixing these issues, I renamed the various functions, fields and pass to talk about "EHCont Targets" instead of "EHCont Guardret". This, unfortunately, resulted in a lot of churn for the MIR parser tests.
> 
> Would it be better if we split this as an NFC patch?

Sure, sounds like a good idea.

> > 2. The SEH personality requires that the basic block containing the `catchpad` is the target, not the `catchret`.
> 
> Can you provide more details about the reasoning. This was implemented by @arlosi from MS. Although I reviewed it, I don't understand the difference between one way and another.

I'm aware of that work: Arlo is on my team. EHCont was originally added to LLVM to handle Rust, which always uses the C++ Exception personality (`MSVC_CXX`) and was never tested against SEH. @namazso is correct: in SEH the unwinder will jump to the `ExceptionHandler` address from the unwind table, hence that needs to be marked as safe in the EHCont metadata.

https://github.com/llvm/llvm-project/pull/129612