A shocking number of LLSingleton subclasses had public constructors -- and in
several instances, were being explicitly instantiated independently of the
LLSingleton machinery. This breaks the new LLSingleton dependency-tracking
machinery. It seems only fair that if you say you want an LLSingleton, there
should only be ONE INSTANCE!

Introduce LLSINGLETON() and LLSINGLETON_EMPTY_CTOR() macros. These handle the
friend class LLSingleton<whatevah>;
and explicitly declare a private nullary constructor.

To try to enforce the LLSINGLETON() convention, introduce a new pure virtual
LLSingleton method you_must_use_LLSINGLETON_macro() which is, as you might
suspect, defined by the macro. If you declare an LLSingleton subclass without
using LLSINGLETON() or LLSINGLETON_EMPTY_CTOR() in the class body, you can't
instantiate the subclass for lack of a you_must_use_LLSINGLETON_macro()
implementation -- which will hopefully remind the coder.

Trawl through ALL LLSingleton subclass definitions, sprinkling in
LLSINGLETON() or LLSINGLETON_EMPTY_CTOR() as appropriate. Remove all explicit
constructor declarations, public or private, along with relevant 'friend class
LLSingleton<myself>' declarations. Where destructors are declared, move them
into private section as well. Where the constructor was inline but nontrivial,
move out of class body.

Fix several LLSingleton abuses revealed by making ctors/dtors private:

LLGlobalEconomy was both an LLSingleton and the base class for
LLRegionEconomy, a non-LLSingleton. (Therefore every LLRegionEconomy instance
contained another instance of the LLGlobalEconomy "singleton.") Extract
LLBaseEconomy; LLGlobalEconomy is now a trivial subclass of that.
LLRegionEconomy, as you might suspect, now derives from LLBaseEconomy.

LLToolGrab, an LLSingleton, was also explicitly instantiated by
LLToolCompGun's constructor. Extract LLToolGrabBase, explicitly instantiated,
with trivial subclass LLToolGrab, the LLSingleton instance.

(WARNING: LLToolGrabBase methods have an unnerving tendency to go after
LLToolGrab::getInstance(). I DO NOT KNOW what should be the relationship
between the instance in LLToolCompGun and the LLToolGrab singleton instance.)

LLGridManager declared a variant constructor accepting (const std::string&),
with the comment:
// initialize with an explicity grid file for testing.
As there is no evidence of this being called from anywhere, delete it.

LLChicletBar's constructor accepted an optional (const LLSD&). As the LLSD
parameter wasn't used, and as there is no evidence of it being passed from
anywhere, delete the parameter.

LLViewerWindow::shutdownViews() was checking LLNavigationBar::
instanceExists(), then deleting its getInstance() pointer -- leaving a
dangling LLSingleton instance pointer, a land mine if any subsequent code
should attempt to reference it. Use deleteSingleton() instead.

~LLAppViewer() was calling LLViewerEventRecorder::instance() and then
explicitly calling ~LLViewerEventRecorder() on that instance -- leaving the
LLSingleton instance pointer pointing to an allocated-but-destroyed instance.
Use deleteSingleton() instead.