Switch nearly all of the use of the UnwindPlan::Row's to go through

a shared pointer to ease some memory management issues with a patch
I'm working on.

The main complication with using SPs for these objects is that most
methods that build up an UnwindPlan will construct a Row to a given
instruction point in a function, then add additional regsaves in
the next instruction point to that row and push it again.  A little
care is needed to not mutate the previous instruction point's Row
once these are switched to being held behing shared pointers.

llvm-svn: 160214