MAINT-1175: Properly pass LLRegistry's COMPARATOR to underlying map.

Although LLRegistry and LLRegistrySingleton have always defined a COMPARATOR
template parameter, it wasn't used for the underlying map. Therefore every
type, including any pointer type, was being compared using std::less. This
happens to work most of the time -- but is tripping us up now.
Pass COMPARATOR to underlying std::map. Fix a couple minor bugs in
LLRegistryDefaultComparator (never before used!). Specialize for const char*.
Remove CompareTypeID and LLCompareTypeID because we now actively forbid using
LLRegistry<std::type_info*, ...>; remove only known reference
(LLWidgetNameRegistry definition).