Handle "--" explicitly in the driver

Anything that comes after -- is treated as an input file. This
used to be handled automagically by the option parsing library,
but after LLVM r188314, we should handle it ourselves.

No functionality change.
<rdar://problem/14717184>

Improve the documentation for the new target.memory-module-load-level setting, and also return an error when there is no nlist data when appropriate.