SjLj based exception handling unwinding support. This patch is nasty, brutish

and short. Well, it's kinda short. Definitely nasty and brutish.

The front-end generates the register/unregister calls into the SjLj runtime,
call-site indices and landing pad dispatch. The back end fills in the LSDA
with the call-site information provided by the front end. Catch blocks are
not yet implemented.

Built on Darwin and verified no llvm-core "make check" regressions.

llvm-svn: 78625