MAINT-5506: Fix ugly timing bug in llurlentry static initialization.

The problem was that class-static LLUrlEntryParcel::sRegionHost was being
initialized by copying class-static LLHost::invalid. Naturally, these two
statics are initialized in different source files. Since C++ makes no promises
about the relative order in which objects in different object files are
initialized, it seems we hit a case in which we were trying to initialize
sRegionHost by copying a completely uninitialized LLHost::invalid.
In general we might attempt to address such cross-translation-unit issues by
introducing an LLSingleton. But in this particular case, the punch line is
that LLHost::invalid is explicitly constructed identically to a
default-constructed LLHost! In other words, LLHost::invalid provides nothing
we couldn't get from LLHost(). All it gives us is an opportunity for glitches
such as the above.
Remove LLHost::invalid and all references, replacing with LLHost().