RFR: 8313554: Fix -Wconversion warnings for ResolvedFieldEntry

[David Holmes]

On Wed, 2 Aug 2023 19:15:24 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.

I'm still not seeing the pattern, in these Wconversion changes, of when to use checked_cast, when to use static_cast and when to use a raw C-style cast??

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PR Review: https://git.openjdk.org/jdk/pull/15126#pullrequestreview-1560325246