RFR: 8302818: Optimize wrapper sets and immutable sets of Enums [v2]

[Tingjun Yuan]

> Currently, the two subclasses of `java.util.EnumSet` optimize bulk operations when the argument is also a `EnumSet`, but there is no such optimization for wrapper sets (returned by `Collections.unmodifiableSet`, `Collections.synchronizedSet`, etc.) and immutable sets (returned by `Set.of` methods) of `Enum`s.
> 
> This PR introduces optimization classes for these situations.  No public APIs are changed.

Tingjun Yuan has updated the pull request incrementally with one additional commit since the last revision:

  Optimize `EnumSet.copyOf` and `Set.copyOf`

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12498/files
  - new: https://git.openjdk.org/jdk/pull/12498/files/7f704c35..41f4a909

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12498&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12498&range=00-01

  Stats: 27 lines in 2 files changed: 22 ins; 2 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/12498.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12498/head:pull/12498

PR: https://git.openjdk.org/jdk/pull/12498