[DWARFv5] Fix dumper to show the file table starts at index 0.

Emitting the correct (root of compilation) file at index 0 will be
posted for review later; I wanted to get this minor change out of the
way first.

llvm-svn: 324669
GitOrigin-RevId: ceafcd41cf27ba94dbff62710ab03cb22b0d23a0
7 files changed