RFR: [15] JDK-8236539 Relative link tags in record javadoc don't resolve

[Jonathan Gibbons]

Please review an interesting but trivial fix for an issue that was 
reported on amber-dev
a while back.

On JDK 14, the test case that was provided executed as described, 
reporting incorrect diagnostics.

On JDK 15, it crashed, caused by a missing 'case RECORD' which is fixed 
here.
Once the crash was addressed, the test case worked as it should, without 
giving
incorrect diagnostics, so the original underlying problem appears to 
have gone away.

The source fix is trivial. The test is just the test case that was 
provided, wrapped up
in our JavadocTester framework.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8236539
Webrev: http://cr.openjdk.java.net/~jjg/8236539/webrev.00/