Eliminate unused variable warnings in panic and unreachable macros

[zhassan-aws]

Description of changes:

Applying the same trick used in #1561 to eliminate spurious unused variable warnings for variables that are exclusively used as arguments in the panic or unreachable macros, both of which Kani provides an overridden version.

Resolved issues:

Resolves #1582

Call-outs:

Testing:

• How is this change tested? Added one test

• Is this a refactor change? No

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

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