RFR JDK-8246199: 'permits' is a restricted identifier

[Vicente Romero]

Please review fix for [1] at [2], `permits` is a restricted identifier 
according to the sealed classes spec [3]. This patch will sync the 
compiler with that part of the spec,

Thanks,
Vicente

[1] https://bugs.openjdk.java.net/browse/JDK-8246199
[2] http://cr.openjdk.java.net/~vromero/8246199/webrev.00/
[3] 
http://cr.openjdk.java.net/~gbierman/jep360/jep360-20200526/specs/sealed-classes-jls.html