Fix linux argument completion with for "--" options (llvm.org/bugs/pr14425)

Patch by Yacine Belkadi!

When __GLIBC__ is defined, optind gets initialized to 0. So for the first parsed
option, parse_start is 0, too. If this option has no argument (Like "--continue"
of "process attach"), then the position stored is 0, instead of 1. This prevents
the completion later on in Options::HandleOptionCompletion() because the opt_pos
doesn't match the cursor_index.

Fix that by getting the option's position from the value of optind, as it's done
for the other types of options.

Re-enable test_process_attach_dash_dash_con() on Linux.

No regressions detected on Mac OS X (in TestCompletion.py)