[yaml2obj] - Add a technical prefix for each unnamed chunk.

This change does not affect the produced binary.

In this patch I assign a technical suffix to each section/fill
(i.e. chunk) name when it is empty. It allows to simplify the code
slightly and improve error messages reported.

In the code we have the section to index mapping, SN2I, which is
globally used. With this change we can use it to map "empty"
names to indexes now, what is helpful.

Differential revision: https://reviews.llvm.org/D79984