RFR: 8285364: Remove REF_ enum for java.lang.ref.Reference [v4]

[Albert Mingkun Yang]

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

Albert Mingkun Yang has updated the pull request incrementally with two additional commits since the last revision:

 - comment
 - Rework reference type initialization
   
   Signed-off-by: Albert Yang <albert.m.yang at oracle.com>

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8332/files
  - new: https://git.openjdk.java.net/jdk/pull/8332/files/73f80ea4..e4f6a66a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8332&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8332&range=02-03

  Stats: 57 lines in 7 files changed: 50 ins; 4 del; 3 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