RFR: 8259214: MetaspaceClosure support for Arrays of MetaspaceObj

[Ioi Lam]

Currently, `MetaspaceClosure::push` supports only the following variants:

MetaspaceClosure* it = ...;
Klass* o = ...; it->push(&o);
Array<int>* a1 = ...; it->push(&a1);
Array<Annotation*>* a2 = ...; it->push(&a2);

In Valhalla, support is needed for the following variant (Annotation is a subtype of MetaspaceObj):

Array<Annotation>* a3 = ...; it->push(&a3);

This change will allow CDS to make a copy of this array, as well as relocating all the pointers embedded in the elements of this array. See a test case in test_metaspaceClosure.cpp.

I also cleaned up the code (with help from @kimbarrett) to use SFINAE to dispatch from `MetaspaceClosure::push` to the different subtypes of `MetaspaceClosure::Ref`.

-------------

Commit messages:
 - 8259214: MetaspaceClosure support for Arrays of MetaspaceObj

Changes: https://git.openjdk.java.net/jdk/pull/1995/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1995&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259214
  Stats: 264 lines in 2 files changed: 209 ins; 31 del; 24 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1995.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1995/head:pull/1995

PR: https://git.openjdk.java.net/jdk/pull/1995