Fix issue #13354 - Use a generic Variant for passing opIndex/opIndexAssign arguments.

Also improves the error message when an incompatible index or value type is used by printing the actual type ID instead of "void[]" in the thrown exception.
Merge pull request #2453 from s-ludwig/issue_13354

Fix issue #13354 - Use a generic Variant for passing opIndex/opIndexAssign arguments.