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

[Reid Kleckner via llvm-commits]

rnk wrote:

As high level feedback on the rename, I worry that "ehcont" is opaque jargon. I correctly guessed that it stands for "exception handling continuation", but I don't assume most readers will get that. LLVM definitely doesn't subscribe to the [long descriptive naming philosophy](https://wiki.c2.com/?VeryLongDescriptiveNamesThatProgrammingPairsThinkProvideGoodDescriptions), so I think it's best to split the difference and use the EHCont variable names as you have them, but please update the comments to spell out "EH continuation(s)" to help readers who aren't familiar with Windows EH tech.

In some ways, these points are similar to landing pads, but we've already overloaded "pad" as a concept to include funclet entry blocks. I guess an LLVM "pad" is any basic block that can be the target of a runtime-mediated control flow transfer.

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