Make LLSD diagnostic methods conditional on LLSD_DEBUG_INFO.

This establishes that there are no viewer-side unit tests relying on these
methods. The point is to try to clean up the LLSD public API. In the same
vein, remove from LLSD public API a diagnostic method which is nothing more
than an implementation detail for the corresponding LLSD::Impl method. The
same effect can be achieved by making LLSD::Impl a friend of LLSD, moving the
method with the messy signature (classic-C arrays!) into LLSD::Impl itself.