[jdk17u-dev] RFR: 8313678: SymbolTable can leak Symbols during cleanup

[Oli Gillespie]

Backporting fix for symbol table leak from https://bugs.openjdk.org/browse/JDK-8313678.

Not totally clean:

* places where 17 uses NULL versus nullptr in tip, trivial fix
* symbolTable uses `Symbol**` in 17 instead of `Symbol*`, also easy fix
* dictionary.cpp is not a concurrent hash table in 17 (missing 8292680), so no need to modify it
* finalizerService, g1CardSet, DynamicSharedSymbols test don't exist in 17

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Commit messages:
 - Backport 4b2703ad39f8160264eb30c797824cc93a6b56e2

Changes: https://git.openjdk.org/jdk17u-dev/pull/1674/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1674&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313678
  Stats: 66 lines in 7 files changed: 43 ins; 12 del; 11 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/1674.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/1674/head:pull/1674

PR: https://git.openjdk.org/jdk17u-dev/pull/1674