Elusive arm64+greedy bug

[xclerc]

There is seeming a miscompilation with
the arm64+greedy configuration, as was
seen in #3496 and #3561.

The failure is now gone, but the symptom
was this assertion failing. It is not clear
whether greedy is actually part of the
problem. It could simply be a very rare
bug; the failure was triggered by adding
(but not even using) a field to a type not
even related to warning handling.

(Note: while the failure was consistent
on the CI, it was intermittent locally.)

[bclement-ocp]

Happened without greedy in #3560 here.

[bclement-ocp]

I got a compile loop going on an arm64 machine with #3561 in the greedy configuration with different random seeds (for the Int_ids hash) that will be running over the weekend — hopefully we will get a consistent reproducer out of it.