Disable dead variant removal for #[repr(C)] enums.

See https://github.com/rust-lang/unsafe-code-guidelines/issues/500.

compiler/rustc_abi/src/lib.rs

@@ -1401,7 +1401,7 @@ pub enum Variants<FieldIdx: Idx, VariantIdx: Idx> {
     /// Single enum variants, structs/tuples, unions, and all non-ADTs.
     Single { index: VariantIdx },
 
-    /// Enum-likes with more than one inhabited variant: each variant comes with
+    /// Enum-likes with more than one variant: each variant comes with
     /// a *discriminant* (usually the same as the variant index but the user can
     /// assign explicit discriminant values). That discriminant is encoded
     /// as a *tag* on the machine. The layout of each variant is