[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