Options: Add new option kind that consumes remaining arguments

This adds KIND_REMAINING_ARGS, a class of options that consume
all remaining arguments on the command line.

This will be used to support /link in clang-cl, which is used
to forward all remaining arguments to the linker.

It also allows us to remove the hard-coded handling of "--",
allowing clients (clang and lld) to implement that functionality
themselves with this new option class.

Differential Revision: http://llvm-reviews.chandlerc.com/D1387