Mark SingleVariant as repr(u8) in c-style-enum

I should rather properly fix debuginfo but I have no clue how to do that.

Author: nox

src/test/debuginfo/c-style-enum.rs

@@ -151,6 +151,7 @@ enum ManualDiscriminant {
 }
 
 #[derive(Copy, Clone)]
+#[repr(u8)]
 enum SingleVariant {
     TheOnlyVariant
 }