[Driver] Add `--` to some test clang-cl command lines. (#70055)

If clang/test/Driver/cl-offload.cu is run on Unix in a directory whose
absolute pathname starts with `/w`, such as the `/workspace` used by at
least some Jenkins CI setups, then the file name on the clang command
line is misinterpreted as some kind of MSVC warning-control option, and
ignored by the catch-all `_SLASH_w` option in Options.td.

Other clang-cl tests take care to put a `--` before the input file name,
to force clang to treat it as a filename even if it starts with a / and
accidentally looks like a cl option. Do the same here.