[DWARF] Why epilogue_begin was not emitted?

We'll be adding this (OpenVMS needs to know about epilogue boundaries to
handle asynchronous exceptions in an epilogue). We are currently
out-of-tree but will be rebasing next year and will then offer up what
we have done.

Epilogue support was attempted at some point, but Francis told me that
it had problems with shrink-wrapping (I think that was the reason).


I might be wrong but I think I was mentioning adding CFI emission support in epilogues, which should be working on X86: https://reviews.llvm.org/rL330706.