RFR: 8313564: Fix -Wconversion warnings in classfile code [v5]

[Coleen Phillimore]

> This patch fixes various -Wconversion warnings in classfile code.  I broke the change into commits so the changes are easier to see.
> Tested with tier1-4.

Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:

  Revert items_count change to int.  Add check for items_count overflow in SymbolTable.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15111/files
  - new: https://git.openjdk.org/jdk/pull/15111/files/4019e4ed..a092d427

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15111&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15111&range=03-04

  Stats: 13 lines in 2 files changed: 3 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/15111.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15111/head:pull/15111

PR: https://git.openjdk.org/jdk/pull/15111