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.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@188316 91177308-0d34-0410-b5e6-96231b3b80d8