I've tested this implementation on x86-64 to ensure that it works. All
libc++abi tests pass, as do all libc++ exception-related tests. ARM
and ARM64 still remain to be implemented (@compnerd?).
Special thanks to KJK::Hyperion for his excellent series of articles on
how EH works on x86-64 Windows. You can find it here:
http://www.nynaeve.net/?p=99. (Seriously, check it out. It's awesome.)
I'm actually not sure if this should go in as is. I particularly don't
like that I duplicated the UnwindCursor class for this special case.