Store end positions directly in JCTree

[cushon]

This is a prototype of storing end positions in a field in JCTree, instead of in a separate EndPosTable.

See this compiler-dev@ thread.

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Progress

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Update a local copy of the PR:
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$ git pr checkout 28506

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https://git.openjdk.org/jdk/pull/28506.diff

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[cushon]

#28610