Options: explicit handling of --

Clients of the option parsing library should handle it explicitly
using a KIND_REMAINING_ARGS option.

Clang and lld have been updated in r188316 and r188318, respectively.

Also fix -Wsign-compare warning in the option parsing test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188323 91177308-0d34-0410-b5e6-96231b3b80d8