Now that we have unit tests that require assertion failure if you try to
delete an LLInstanceTracker subclass instance with an iterator loose, having
llassert() "sometimes" compile away (whimsically, depending on platform as
well as build type!) makes those tests fail. Use llassert_always() instead.

The recent class-static LLInstanceTracker::instance_iter and key_iter
reference count is intended to guard against deleting an instance of an
LLInstanceTracker subclass during iteration. Add tests for that functionality.

Generalize the notion of getting some chunk of "static" storage: introduce
LLInstanceTrackerBase::getStatic() template method. Define StaticData struct
containing the InstanceMap (or InstanceSet, for that specialization) along
with the S32 that caused the VS2010 linker so much grief. Completely eliminate
that S32 as an actual class-static member, qualifying all references with the
struct returned by getStatic().
In LLInstanceTrackerBase::getInstances(), use one std::map lookup instead of
three.

reviewed by Stone

EXP-1160 FIX Cannot drag/drop items from inventory into the contents of a prim in latest Mesh build 239475
brought back some old logic for dispatching to child folder as drag and drop fallback

Added comment where sidebar block list is opened.