[Bug ada/124107] 13.11.5(9.1/5) is not followed - finalization removes subpools from pools

[liam at liampwll dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124107

--- Comment #2 from Liam Powell <liam at liampwll dot com> ---
The bug still exists without Unchecked_Access, the RM is clear that
finalization of the subpool must remove it from the pool. Unchecked_Access is
just when this bug can cause the major issues. There's unfortunately no way to
do what I'm doing in the linked PR without Unchecked_Access, but it should all
be safe according to the RM.

I also bisected this to g:48d7a599ecd141f7936deff6170dd5199edb2d98.