[DWARFv5] Number the line-table's directory array correctly.

The compilation directory has always been #0, but as of DWARF v5 it is
explicitly listed in the line-table section instead of implicitly
being a reference to the compile_unit DIE's DW_AT_comp_dir attribute.
This means the dumper should number the dumped array starting with 0
or 1 depending on the DWARF version of the line table.

References in the generated DWARF are correct, it's just the dumper
that was wrong.  Also some assembler-coded tests were similarly
confused about directory numbers.

llvm-svn: 322884