RFR: 8308655: Narrow types of ConstantPool and ConstMethod returns [v3]

[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.

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

  Matias's comments.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14092/files
  - new: https://git.openjdk.org/jdk/pull/14092/files/7398e225..cb9458c9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14092&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14092&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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