If a mutex interface is split in inheritance chain, e.g. struct mutex
has `unlock` and inherits `lock` from __mutex_base then calls m.lock()
and m.unlock() have different "this" targets: m and the __mutex_base of
m, which used to confuse the `ActiveCritSections` list.
Taking base region canonicalizes the region used to identify a critical
section and enables search in ActiveCritSections list regardless of
which class the callee is the member of.
This likely fixes#104241
CPP-5541
