Track conditional deletions in the semantic model

[charliermarsh]

Summary

Given del X, we'll typically add a BindingKind::Deletion to X to shadow the current binding. However, if the deletion is inside of a conditional operation, we won't, as in:

def f():
    global X

    if X > 0:
        del X

We will, however, track it as a reference to the binding. This PR adds the expression context to those resolved references, so that we can detect that the X in global X was "assigned to".

Closes #10397.

[github-actions]

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