Fix not setting offset on DexItemBasedConstString replacement

If this is not done it will cause errors when computing offset in the
JumboStringRewriter.

Change-Id: I321e2714272f0a907d65bf29062c1f67455e0626
diff --git a/src/main/java/com/android/tools/r8/naming/IdentifierMinifier.java b/src/main/java/com/android/tools/r8/naming/IdentifierMinifier.java
index 61695f1..e670925 100644
--- a/src/main/java/com/android/tools/r8/naming/IdentifierMinifier.java
+++ b/src/main/java/com/android/tools/r8/naming/IdentifierMinifier.java
@@ -170,7 +170,9 @@
                           appInfo.definitionFor(cnst.getItem().asDexType()),
                           cnst.getClassNameComputationInfo()))
                   : lens.lookupName(cnst.getItem(), appInfo.dexItemFactory);
-          instructions[i] = new ConstString(cnst.AA, replacement);
+          ConstString constString = new ConstString(cnst.AA, replacement);
+          constString.setOffset(instruction.getOffset());
+          instructions[i] = constString;
         }
       }
     } else {