Add a new 'target modules show-unwind' command to show the different

UnwindPlans for a function.  This specifically does not use any
previously-generated UnwindPlans so if any logging is performed
while creating the UnwindPlans, it will be repeated.  This is
useful for when an lldb stack trace is not correct and you want
to gather diagnostic information from the user -- they can do 
log enable -v lldb unwind, image show-unwind of the function, and
you'll get the full logging as the UnwindPlans are recreated.

llvm-svn: 160095