RFR: 8341907: javac -Xlint should ignore /// on first line of source file

[Pavel Rappo]

Please review this PR to exempt the [trick that JBang uses]() from the "dangling comment" lint introduced in JDK 23. For more information see this PR's JBS issue and its comment section.

The fix makes sure that the warning is not issued if it relates to a leading `///` comment. For simplicity and similarity with the shebang construct, the comment should start from the first character of the file, and not a more permissive first non-whitespace character of the file.

Since I'm not an expert in `java.compiler`, I'm unsure if my code intercepts warning in the most appropriate layer. Please double-check that.

Skimming through tests which I used for inspiration, `test/langtools/tools/javac/danglingDocComments`, I was unsure why they compile itself first without any `/ref=` construct. A test I introduced here does not do this; am I missing something?

[trick that JBang uses]: https://www.jbang.dev/documentation/guide/latest/usage.html

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/21923/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21923&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341907
  Stats: 55 lines in 5 files changed: 54 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21923.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21923/head:pull/21923

PR: https://git.openjdk.org/jdk/pull/21923