MAINT-5232: Add DEBUG logging to LLSingleton operations.

Specifically, log as LLSingleton captures inter-Singleton dependencies. Also
log cleanupAll() calls to cleanupSingleton() and deleteAll() calls to
deleteSingleton(), since they happen in an implicitly-determined order. But do
not log anything during the implicit LLSingletonBase::deleteAll() call
triggered by the runtime destroying the last LLSingleton's static data. That's
too late in the run; even std::cerr might already have been destroyed!