Fix [override] error with no line number when argument node has no line number

[brianschubert]

Refs #18115

When a parameter type in a method override is incompatible with the parameter type in the supertype definition, mypy emits an error using the Argument node as the context.

However, sometimes the the Argument node doesn't have a line number set, causing the error message to have no associated line number. This happens with the __replace__ methods created in the dataclass plugin, which have line numbers set on the FuncDef nodes, but no line numbers set on the individual argument nodes.

This PR fixes the missing line number in the error by falling-back to the FuncDef line number when a line number isn't set on the Argument node.

(As an alternative fix, we could add line numbers to the Argument nodes in the dataclass plugin, but that looks like a more complicated change since multiple methods would be affected).

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[JukkaL]

Thanks for the fix!