[code-reflection] RFR: Fix unbox invocation

[Paul Sandoz]

The compiler generated incorrect models for unboxing:
1. The unboxing method is a virtual method on the box class, not a static method (as is the case for boxing).
2. When primitive casting to a box we need to use the boxed type as the referencing class of the invocation not `Object`.

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Commit messages:
 - Fix unbox invocation.

Changes: https://git.openjdk.org/babylon/pull/41/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=41&range=00
  Stats: 21 lines in 4 files changed: 3 ins; 0 del; 18 mod
  Patch: https://git.openjdk.org/babylon/pull/41.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/41/head:pull/41

PR: https://git.openjdk.org/babylon/pull/41