Write empty pchs if the user asks to and consider this in TCling
Do not generate fwd decl payload nor autoparse keys when pcm is empty.

This allows to gain memory and runtime at library load. The improvement
has not a tremendous impact on long running jobs but it is clearly
visible at startup. If the payload is not present parsing is avoided.
Re-enable injection of autoparse keys for non-stage 1, non-in-pch dictionaries