RFR: 8349512: Duplicate PermittedSubclasses entries with doclint enabled

[Vicente Romero]

On Fri, 7 Feb 2025 00:31:36 GMT, Liam Miller-Cushon <cushon at openjdk.org> wrote:

> This change avoids emitting duplicate `PermittedSubclasses` entries when `-Xdoclint` is enabled.
> 
> `-Xdoclint` causes some trees to be re-attributed, and the second attribution was resulting in permitted subclasses being added again, and those side effects were not discarded. This fix causes `addPermittedSubclass` to not insert duplicate entries.

lgtm

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Marked as reviewed by vromero (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/23507#pullrequestreview-2602268447