Handle both homogeneous and inhomogeneous clone_from.

Author: cjgillot

compiler/rustc_index/src/bit_set.rs

@@ -1078,8 +1078,12 @@ impl<T> Clone for BitSet<T> {
     }
 
     fn clone_from(&mut self, from: &Self) {
-        assert_eq!(self.domain_size, from.domain_size);
-        bitwise(&mut self.words, &from.words, |_, b| b);
+        if self.domain_size == from.domain_size {
+            bitwise(&mut self.words, &from.words, |_, b| b);
+        } else {
+            self.domain_size = from.domain_size;
+            self.words.clone_from(&from.words);
+        }
     }
 }