RFR: 8315588: JShell does not accept underscore from JEP 443 even with --enable-preview

[Aggelos Biboudis]

The current status is that we get an `UNKNOWN` status for those incomplete snippets that involve underscore declaring local variables or patterns. Those should have been classified as `DEFINITELY_INCOMPLETE`. This process takes place in `SourceCodeAnalysisImpl` and `CompletenessAnalyzer` and the processing that takes place regarded `_` as a location of kind `XERR0`. It would be correct now to classify it with `XDECL1`”.

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Commit messages:
 - 8315588: JShell does not accept underscore from JEP 443 even with --enable-preview

Changes: https://git.openjdk.org/jdk/pull/15765/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15765&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315588
  Stats: 80 lines in 2 files changed: 78 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/15765.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15765/head:pull/15765

PR: https://git.openjdk.org/jdk/pull/15765