fix: annotated argument's var node type is explicit, not inferred

[bzoracler]

Fixes #17216

During conversion from a standard library AST to the mypy AST, Var nodes were being created inside Argument nodes without acknowledging the presence of a type annotation, leading to the Var node's type as being always set as inferred:

mypy/mypy/nodes.py

Line 988 in fb31409

self.is_inferred = self.type is None

This causes an error at

mypy/mypyc/irbuild/expression.py

Lines 161 to 164 in fb31409

	and expr.node.is_inferred
	):
	builder.error(
	'Local variable "{}" has inferred type None; add an annotation'.format(

The fix simply acknowledges any presence of a type annotation, so the type of the relevant Var node is no longer considered inferred if an annotation is present.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

spark (https://github.com/apache/spark)
+ python/pyspark/core/rdd.py:1662: error: Function does not return a value (it only ever returns None)  [func-returns-value]
+ python/pyspark/core/rdd.py:1664: error: Unused "type: ignore" comment  [unused-ignore]

[bzoracler]

Primer outputs look OK:

• First warning is correct
• Second warning (overloads) stems from an unrelated bug:

  from collections.abc import Callable
  
  f: Callable[[object], None] = lambda _: None
  
  a = f("a")  # Correct error: Function does not return a value (it only ever returns None) [func-returns-value]
  reveal_type(a)  # Bug: revealed type is `Any` (should be `None`)

[hauntsaninja]

Thanks!