Add a little sketch of a program that can extract unwind

information from the compact unwind section used on darwin
for exception handling, and dump that information..

The UNWIND_X86_64_MODE_RBP_FRAME and UNWIND_X86_64_MODE_DWARF
entries look to be handled correctly.

UNWIND_X86_64_MODE_STACK_IMMD and UNWIND_X86_64_MODE_STACK_IND 
are still a work in progress.

Only x86_64 is supported right now.  Given that this is an
experiment in parsing the section contents, I don't expect to
add other architectures; they are trivial variations on this
arch.  There exists a real dumper included in the Xcode tools, 
unwinddump.

llvm-svn: 222127