RFR: 8256516: Simplify clearing References [v2]

[Kim Barrett]

> Please review this simplification of jlr.Reference clearing by VM code.
> 
> The function java_lang_ref_Reference::set_referent_raw was being used to
> clear the referent of Reference objects, and only for that purpose.  This
> change replaces that function with java_lang_ref_Reference::clear_referent,
> which is much more obvious in intent.  That change is then percolated up
> through callers in the obvious way.
> 
> Testing:
> mach5 tier1

Kim Barrett has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge branch 'master' into simplify
 - replace set_referent with clear_referent

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1286/files
  - new: https://git.openjdk.java.net/jdk/pull/1286/files/84dc63e4..74a67a1a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1286&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1286&range=00-01

  Stats: 8917 lines in 316 files changed: 5071 ins; 2346 del; 1500 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1286.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1286/head:pull/1286

PR: https://git.openjdk.java.net/jdk/pull/1286