SL-10297: Make LLSingletonBase::llerrs() et al. runtime variadic.

Instead of accepting a fixed list of (const char* p1="", etc.), accept
(std::initializer_list<std::string_view>). Accepting a
std::initializer_list<T> in your parameter list allows coding (e.g.)
func({T0, T1, T2, ... });
-- in other words, you can pass the initializer_list a brace-enclosed list of
an arbitrary number of instances of T.

Using std::string_view instead of const char* means we can pass *either* const
char* or std::string. string_view is cheaply constructed from either, allowing
uniform treatment within the function.

Constructing string_view from std::string simply extracts the pointer and
length from the std::string.

Constructing string_view from const char* (e.g. a "string literal") requires
scanning the string for its terminating nul byte -- but that would be
necessary even if the scan were deferred until the function body. Since
string_view stores the length, the scan still happens only once.