RFR: 8311077: Fix -Wconversion warnings in jvmti code.

[Coleen Phillimore]

Please review change for mostly fixing return types in the constant pool and metadata to fix -Wconversion warnings in JVMTI code.
Tested with tier1-4.

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Commit messages:
 - Fix -Wconversion warnings in jvmti code.

Changes: https://git.openjdk.org/jdk/pull/14710/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14710&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311077
  Stats: 100 lines in 16 files changed: 4 ins; 2 del; 94 mod
  Patch: https://git.openjdk.org/jdk/pull/14710.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14710/head:pull/14710

PR: https://git.openjdk.org/jdk/pull/14710