RFR: 8256516: Simplify clearing References

[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

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Commit messages:
 - replace set_referent with clear_referent

Changes: https://git.openjdk.java.net/jdk/pull/1286/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1286&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256516
  Stats: 32 lines in 7 files changed: 9 ins; 10 del; 13 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