[Option] Support special argument "--"

Many command line option implementations, including getopt_long and our
Support/CommandLine.cpp, support `--` as an end-of-option indicator. All
the subsequent arguments are then treated as positional arguments.

D1387 added KIND_REMAINING_ARGS and 76ff1d91 dropped special handling of `--`.
Users need to add `def DASH_DASH : Option<["--"], "", KIND_REMAINING_ARGS>;` and
append `OPT_DASH_DASH` to the `OPT_INPUT` list., which is not ergonomic.

Restore this feature under an option and modify llvm-strings to utilize the
feature as an example. In the future, we probably should enable this feature by
default and exclude some tools that handle `DASH_DASH` differently (clang,
clang-scan-deps, etc. I suspect that many are workarounds for LLVMOption not
supporting `--` as a built-in feature).

Reviewed By: serge-sans-paille

Differential Revision: https://reviews.llvm.org/D152286