MCDwarf: Refactor line table handling into a single data structure
This replaces several "compile unit ID -> thing" mappings in favor of
one mapping from CUID to the whole line table structure (files,
directories, and lines).
This is another step along the way to refactoring out reusable
components of line table handling for use when generating debug_line.dwo
for fission type units.
Also, might be a good basis to fold some of this handling down into
MCStreamers to avoid the special case of "One line table when doing asm
printing, line table per CU otherwise" by building it into the different