RFR: 8313554: Fix -Wconversion warnings for ResolvedFieldEntry [v3]

[Matias Saavedra Silva]

> 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:

  Added asserts for flags

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15126/files
  - new: https://git.openjdk.org/jdk/pull/15126/files/da649530..3a0ae21d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15126&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15126&range=01-02

  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15126.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15126/head:pull/15126

PR: https://git.openjdk.org/jdk/pull/15126