[Bug c++/68859] Add a less strict/smarter version of -Wreorder

[scovich at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68859

--- Comment #1 from Ryan Johnson <scovich at gmail dot com> ---
(I would be happy to do some legwork on this if somebody is willing to send a
few pointers by PM. I know the code in gcc/cp/init.c, particularly functions
`perform_member_init` and `sort_mem_initializers` are relevant, but would need
some help figuring out how to traverse trees and pick out uses of member
variables from other members' initializers)