EH: Generate EH clauses to report back to runtime

[JosephTremoulet]

The EH clauses we're generating aren't exactly the same as the pdata that LLVM will be generating; need to override/translate as appropriate.

The plan is to first implement this when just catch handlers are processed in the reader, so the initial implementation can take advantage of that if convenient.

Part of #13.

[JosephTremoulet]

Clearing the Sprint 83 milestone and putting this on hold pending the coming LLVM EH changes. The work-so-far on top of the old model is posted in #606 and microsoft/llvm#43 for reference.

[JosephTremoulet]

This got merged into master with #897.