RFR: 8285364: Remove REF_ enum for java.lang.ref.Reference [v6]

[Albert Mingkun Yang]

> Simple rename and some comments update.
> 
> Test: build

Albert Mingkun Yang has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains seven commits:

 - merge
 - review
 - comment
 - Rework reference type initialization
   
   Signed-off-by: Albert Yang <albert.m.yang at oracle.com>
 - Remove REF_ enum for java.lang.ref.Reference
   
   Signed-off-by: Albert Yang <albert.m.yang at oracle.com>
 - review
 - ref-rename

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Changes: https://git.openjdk.java.net/jdk/pull/8332/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8332&range=05
  Stats: 119 lines in 14 files changed: 81 ins; 31 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8332.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8332/head:pull/8332

PR: https://git.openjdk.java.net/jdk/pull/8332