[DWARFYAML][debug_info] Replace 'InitialLength' with 'Format' and 'Length'.

'InitialLength' is replaced with 'Format' (DWARF32 by default) and 'Length' in this patch.
Besides, test cases for DWARFv4 and DWARFv5, DWARF32 and DWARF64 is
added.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D82622