RFR: 8302603: Use Set.of in java.nio.charset.Charset [v2]

[Glavo]

> A small patch, using `Set.of` instead of `Collections.unmodifiableSet()` of `Charset#aliasSet`.
> 
> The alias should not be null, and the aliasSet is an immutable copy, so it is safe to use `Set.of`.
> 
> I have investigated all charsets, most of charsets have no more than 4 aliases, and the maximum is only 17.
> 
> In this case, the performance difference between immutable sets and HashSet can be ignored, and the memory footprint of immutable sets is smaller.
> 
> I need someone to help me open an Issue on JBS. Thank you very much.

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

  update

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12570/files
  - new: https://git.openjdk.org/jdk/pull/12570/files/f28a3f1b..0eea8b8c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12570&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12570&range=00-01

  Stats: 7 lines in 1 file changed: 3 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/12570.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12570/head:pull/12570

PR: https://git.openjdk.org/jdk/pull/12570