Recommit: CFGDiff: Simplify/common the begin/end implementations to use a common range helper""

(would be nice to revisit the CFG traits and change them to use ranges
rather than begin/end - if anyone wants to do that refactor)

Also use more auto because writing the names of range utilty iterators
isn't helping readability here - they're sort of implementation details
for the most part, especially once you nest a few different filtering
and adapting iterators.

The fix (shooting from the hip since I couldn't reproduce this locally)
was to capture by value in a lambda used in a filtering iterator -
because the iterator would persist beyond the lifetime of the function
(as the iterators are returned to callers).

Originally committed in 79a7ed92.
This was reverted in 4a7f2032.