This allows avoiding conflicts between paths that begin with the same chars as some llvm-rc options (which can be used with either slashes or dashes).
It turned out that handling of -- for separating options from input paths isn't a built-in feature in the Options library, but within e.g. clang it's handled manually as part of a loop that iterates over input arguments.
Should this be " -- "? I don't think llvm-rc.exe foo--bar should trigger this, that's one argument that happens to have 2 dashes.