RFR: 8242217: Shenandoah: Enable GC mode to be diagnostic/experimental and have a name

[Roman Kennke]

Currently, only heuristics can be flagged experimental or diagnostic, or
even have their name reported on -Xlog:gc+init. GC modes should have
those properties too.


Issue:
https://bugs.openjdk.java.net/browse/JDK-8242217
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8242217/webrev.00/

Testing: hotspot_gc_shenandoah

Good?
Roman