RFR: JDK-8315458 Implementation of Implicitly Declared Classes and Instance Main Method (Second Preview)

[Jim Laskey]

Address changes from JEP 445 to JEP 463.

- Move from a SYNTHETIC unnamed class to a MANDATED implicit class.

- Don't mark class on read.

- Remove reflection and annotation processing related to unnamed classes.

- Simplify main method search.

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Commit messages:
 - Implicit classes and instance main methods

Changes: https://git.openjdk.org/jdk/pull/16461/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16461&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315458
  Stats: 344 lines in 36 files changed: 12 ins; 231 del; 101 mod
  Patch: https://git.openjdk.org/jdk/pull/16461.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16461/head:pull/16461

PR: https://git.openjdk.org/jdk/pull/16461