[code-reflection] RFR: Add common supertype for type-variable owners

[Maurizio Cimadamore]

This is a followup of https://git.openjdk.org/babylon/pull/64

The goal is to define a common supertype for type-variable owner, namely `Type>

This is a sealed interface implemented by both `MethodRef` and `ClassType`.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/babylon/pull/66/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=66&range=00
  Stats: 30 lines in 8 files changed: 7 ins; 9 del; 14 mod
  Patch: https://git.openjdk.org/babylon/pull/66.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/66/head:pull/66

PR: https://git.openjdk.org/babylon/pull/66