RFR: 8313554: Fix -Wconversion warnings for ResolvedFieldEntry [v2]

[Dean Long]

On Thu, 3 Aug 2023 20:53:56 GMT, Matias Saavedra Silva <matsaave at openjdk.org> wrote:

>> The recent change in [JDK-8301996](https://bugs.openjdk.org/browse/JDK-8301996) added more -Wconversion warnings that are addressed in this patch. The aforementioned change has overlooked inconsistencies with the types used by `ResolvedFieldEntry` and the method `fill_in()`. Verified with tier 1-4 tests.
>
> Matias Saavedra Silva has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Change to checked_cast

Marked as reviewed by dlong (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/15126#pullrequestreview-1562056121