Fixespython#12998mypy can't narrow match statements with functions subjects because thecallexpr node is not a literal node. This adds a 'dummy' literal nodethat the match statement visitor can use to do the type narrowing.The python grammar describes the the match subject as a named expressionso this uses that nameexpr node as it's literal.---------Co-authored-by: hauntsaninja <hauntsaninja@gmail.com>

Fixes#18440.Fixes#17230.Fixes#16650. Improves behavior in#14731(but still thinks that match is non-exhaustive, "missing return" falsepositive remains).#16503 did this specifically for `CallExpr`, but that isn't the onlykind of such statements. I propose to expand this for more generalexpressions and believe that a blacklist is more reasonable here: we do**not** want to introduce a temporary name only for certain expressionsthat either are already named or can be used to infer containedvariables (inline tuple/list/dict/set literals).Writing logic to generate a name for every other kind of expressionwould be quite cumbersome - I circumvent this by using a simple counterto generate unique names on demand.---------Co-authored-by: Ivan Levkivskyi <levkivskyi@gmail.com>