[DWP] Fix default for continue-on-cu-index-overflow (#75540)

This is follow up for https://github.com/llvm/llvm-project/pull/71902.
The
default option --continue-on-cu-index-overflow returned an error
--continue-on-cu-index-overflow: missing argument. Changed it so that it
is the
same behavior as other flags like -gsplit-dwarf. Where
--continue-on-cu-index-overflow will default to continue, and user can
set mode
with --continue-on-cu-index-overflow=\<value>.