[code-reflection] RFR: Fix interpreter arg checking

[Paul Sandoz]

A prior update to the interpreter to check its arguments caused some test failures. This is a quick fix to ensure test pass while still ensuring most checks are still in place
1. ensure run time representation of variables (Var) are supported. 
2. allow for primitive conversions, which for now i punt on and just let them through, as the check needs more thought.
3. accept null for any argument, refine later for primitive values.

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Commit messages:
 - Fix interpreter arg checking.

Changes: https://git.openjdk.org/babylon/pull/524/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=524&range=00
  Stats: 9 lines in 2 files changed: 4 ins; 3 del; 2 mod
  Patch: https://git.openjdk.org/babylon/pull/524.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/524/head:pull/524

PR: https://git.openjdk.org/babylon/pull/524