Commit ea9df812 relaxes the requirement
that all LWLocks are in a single array, which broke the LOCK_DEBUG
code for trying to acquiring an LWLock. This is a meatball surgery
attempt at making it at least compile and run, without trying to
reinvent the old usecase.

Minor tweaking of the code is also done to make LOCK_DEBUG cases be
identified identically.

Reported-by: Simon Gao in Github issue #7251
Reviewed-by: Ashwin Agrawal