RFR: 8308655: Narrow types of ConstantPool and ConstMethod returns

[Coleen Phillimore]

This change uses a number of ways to eliminate -Wconversion warnings in the metadata files in the oops directory. 

1. narrow return types to u2 if the accessor is for a field or value that's u2 (u2 is most common for constMethod fields and constant pool indices)
2. Use checked_cast<type> for places where we know the int value is u2 or s2 but propagating these types is too much fan out.
3. Use plain casts where it's obvious that the int value fits in the casted-to type.
4. Moved KlassKind to be contained in Klass to add the Unknown enum value to use instead of -1.
5. Moved the compute_from_signature function into ConstMethod as it sets values in ConstMethod and the parameters are changed in the set functions.  Removed some pass through functions in Method.

Tested with tier1-4.

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Commit messages:
 - merge error
 - Fix ConstantPool types.
 - Refine ConstMethod parameter types and int size checking.

Changes: https://git.openjdk.org/jdk/pull/14092/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14092&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308655
  Stats: 201 lines in 31 files changed: 43 ins; 42 del; 116 mod
  Patch: https://git.openjdk.org/jdk/pull/14092.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14092/head:pull/14092

PR: https://git.openjdk.org/jdk/pull/14092