RFR: 8341176: Permit access to diagnostics for transient snippets

[Jan Lahoda]

JShell permits to create transient snippets using `SourceCodeAnalysis.sourceToSnippets`, for the purpose of alternative UI clients. But, the alternative UI clients might need to get the diagnostics for the snippets, but the existing `JShell.diagnostics(Snippet)` will not work for the transient snippets.

This patch proposes to enhance the `JShell.diagnostics(Snippet)` with an ability to return best-effort diagnostics for these transient snippets.

Please also review the CSR:
https://bugs.openjdk.org/browse/JDK-8341200

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Commit messages:
 - Updating copyright years.
 - 8341176: Permit access to diagnostics for temporary snippets

Changes: https://git.openjdk.org/jdk/pull/21261/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21261&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341176
  Stats: 91 lines in 4 files changed: 84 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/21261.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21261/head:pull/21261

PR: https://git.openjdk.org/jdk/pull/21261