RFR: 8341907: javac -Xlint should ignore /// on first line of source file [v3]

[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 the tests which I used for inspiration, `test/langtools/tools/javac/danglingDocComments`, I was unsure why they compile itself first without any `/ref=` construct. The 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

Pavel Rappo has updated the pull request incrementally with two additional commits since the last revision:

 - Fix a pre-existing doc typo
 - Make the check more specific

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/21923/files
  - new: https://git.openjdk.org/jdk/pull/21923/files/cc9a379d..65ba8dca

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21923&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21923&range=01-02

  Stats: 38 lines in 5 files changed: 35 ins; 1 del; 2 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