[PECOFF] Handle "--" option explicitly

This used to be handled automagically by the option parsing library,
but after LLVM r188314, we should handle it ourselves.

No functionality change, but adds a test.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@188318 91177308-0d34-0410-b5e6-96231b3b80d8