[lldb/Driver] Fix handling on positional arguments

Before the transition to libOption it was possible to specify arguments
for the inferior without -- as long as they didn't start with a dash.

For example, the following invocations should all behave the same:

  $ lldb inferior inferior-arg
  $ lldb inferior -- inferior-arg
  $ lldb -- inferior inferior-arg

This patch fixes that behavior, documents it and adds a test to cover
the different combinations.

Differential revision: https://reviews.llvm.org/D80165