hg: panama/panama/jdk: Fix: jextract crashes with SOE when mutually recursive struct are found

[maurizio.cimadamore at oracle.com]

Changeset: 36451564cca0
Author:    mcimadamore
Date:      2017-08-17 15:26 +0100
URL:       http://hg.openjdk.java.net/panama/panama/jdk/rev/36451564cca0

Fix: jextract crashes with SOE when mutually recursive struct are found
Layout generation walks the innards of a struct eagerly - so cycles need to be broken to prevent crashes.
Solution is to detect the cycle and emit a less sharp layout description (e.g. 'p' instead of 'p:xyz').

! src/jdk.jextract/share/classes/com/sun/tools/jextract/Utils.java