[red-knot] handle synthetic 'self' argument in call-binding diagnostics

[carljm]

Summary

In binding calls, we store an argument index with argument-specific binding errors, so that we can emit the diagnostic on the specific argument node at the call site.

We need to account for the synthetic self argument for methods, or else our argument index is off by one and can result in a panic where we are looking for an argument node that doesn't exist in the call.

Fixes #15360

Test Plan

Added mdtest.

Also verified that with this fix, red-knot can again run on Black codebase without panic.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.