JDK 13 RFR of JDK-8224177: Harden annotation processing framework to irregular behavior from processors

[Joe Darcy]

Hello,

As a complement to the changes in AbstractProcessor done under 
JDK-8146726 , the annotation processing framework in javac could be 
hardened against various kinds of irregular results returned from 
processors. The hardening can include just warning about redundant but 
not strictly incorrect information.

Webrev at:

     http://cr.openjdk.java.net/~darcy/8224177.0/

Most of the code changes in JavacProcessingEnvironment are to guard 
against misbehaving Set implementations returned from Processors; 
perhaps this is an unnecessary degree of caution.

There may be other test updates needed for account for the new warnings 
messages; I'll make any such update once there is review feedback and 
agreement on (what subset of) this change's checks should go back.

Thanks,

-Joe