RFR: JDK-8215510: j.l.c.ClassDesc is accepting descriptors not allowed by the spec

[Vicente Romero]

Please review the fix for bug [1] at [2]. Basically the constants API 
introduced as part of JEP-334 [3] were accepting descriptors and class 
names not allowed by the spec. This implementation fixes several issues 
found by TCK tests on JEP-334,

Thanks,
Vicente
[1] https://bugs.openjdk.java.net/browse/JDK-8215510
[2] http://cr.openjdk.java.net/~vromero/8215510/webrev.00/
[3] https://bugs.openjdk.java.net/browse/JDK-8203252