This is harmless due to the previous checks for > 0, but it is still
confusing for the readers.

Also rename the "sp" suffix (originally intended to mean "Stack
Pointer") to "cfa", as "sp" generally means Shared Pointer.

In most places, the rows are copied anyway (because they are generated
by cumulating modifications) immediately after adding them to the unwind
plans. In others, they can be moved into the unwind plan. This lets us
remove some backflip copies and make `const UnwindPlan` actually mean
something.

I've split this patch into two (and temporarily left both APIs) as this
patch was getting a bit big. This patch covers all the interesting
cases. Part two all about converting "architecture default" unwind plans
from ABI and InstructionEmulation plugins.

My main motivation was trying to understand how the function and whether
the rows need to be (shared) pointers. I noticed that the function
essentially constructs two copies unwind plans in parallel (the second
being the saved_unwind_states).

If we delay the construction of the unwind plan to the end of the
function, then we never need two copies of a single row (we can just
move it into the final result), so we can just use them as value types.
This makes the overall logic of the function stand out better as it
avoids the laborious deep copies of the Row shared pointer.

I've also noticed that a large portion of the function is devoted to
recomputing certain properties of the unwind state (e.g. the
`m_fp_is_cfa` field). Instead of doing that, this patch just
saves/restores them together with the rest of the state.