RFR JDK-14: 8236266: change error message for the case when a class extends j.l.Record

[Vicente Romero]

Please review the patch to fix [1] in JDK 14.  I pushed the code 
initially to 15 when it should have been pushed to 14.  The patch 
applies cleanly.  The fix in JDK 15 can be is at [3] and the review 
thread can be accessed at [4],

Thanks,
Vicente

[1] https://bugs.openjdk.java.net/browse/JDK-8236266
[2] http://cr.openjdk.java.net/~vromero/8236266-jdk14/webrev.00/
[3] https://bugs.openjdk.java.net/browse/JDK-8236279
[4]http://mail.openjdk.java.net/pipermail/compiler-dev/2019-December/014120.html