sccache doesn't support clang-cl on non-Windows

[glandium]

This code:

sccache/src/compiler/msvc.rs

Lines 273 to 282 in 9d13465

	// First convert all `/foo` arguments to `-foo` to accept both styles
	let it = arguments.iter().map(|i| {
	if let Some(arg) = i.split_prefix("/") {
	let mut dash = OsString::from("-");
	dash.push(&arg);
	dash
	} else {
	i.clone()
	}
	});

unconditionally transforms all arguments starting with a / to start with a -, which is fine for both MSVC and clang-cl when they run on Windows, but breaks things when running clang-cl on e.g. Linux.

[glandium]

To make matters more interesting... clang-cl on Linux actually supports / arguments too, so we can't technically brush away things that start with a / as being paths.