add some comments on how this is *supposed* to work.  We don't
need the PreferredEHDataFormat hook, but I have yet-more refactoring to
do before I can zap it.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@77742 91177308-0d34-0410-b5e6-96231b3b80d8
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