RFR: 8292286: Convert PlaceholderTable to ResourceHashtable [v2]

[Coleen Phillimore]

> This change converts the PlaceholderTable to ResourceHashtable.  It's much like the ResolutionErrorTable conversion.  This change has been performance tested with the previous set of testing with no regression.  It's also been tested tier1-3 and 4-7 with the other tables in my patch queue.

Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:

  Use Symbol::maybe increment or decrement refcount.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/9854/files
  - new: https://git.openjdk.org/jdk/pull/9854/files/76db3258..4a0cd646

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9854&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9854&range=00-01

  Stats: 4 lines in 2 files changed: 0 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/9854.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9854/head:pull/9854

PR: https://git.openjdk.org/jdk/pull/9854