MAINT-5232: Make LLError::is_available() depend on both LLSingletons.

LLError machinery depends on two different LLSingletons. Its is_available()
function is primarily for LLSingleton itself to determine whether it is, or is
not, safe to log. Until both of LLError's LLSingletons have been constructed,
attempting to log LLSingleton operations could produce infinite recursion.