Fix clang-cl treating file as a /U arg.

On MacOS, paths will likely start with /Users, but clang-cl treats /U
as an argument, which leads to the source being mistaken for a flag. To
avoid this, we can pass -- before the source to ensure argument parsing
is disabled.

src/lib.rs

@@ -1311,6 +1311,10 @@ impl Build {
         if self.cuda && self.files.len() > 1 {
             cmd.arg("--device-c");
         }
+        // Ensure src is not treated as a clang-cl argument, like /U.
+        if compiler.family == ToolFamily::Msvc { clang_cl: true } {
+            cmd.arg("--");
+        }
         cmd.arg(&obj.src);
         if cfg!(target_os = "macos") {
             self.fix_env_for_apple_os(&mut cmd)?;