constant: Add support for unary negation.

tests/rust/bitflags.rs

@@ -34,6 +34,7 @@ bitflags! {
     pub struct LargeFlags: u64 {
         /// Flag with a very large shift that usually would be narrowed.
         const LARGE_SHIFT = 1u64 << 44;
+        const INVERTED = !Self::LARGE_SHIFT.bits;
     }
 }