Fix let-patterns inside named arguments

[lukaszcz]

• Closes impossible crash #3269
• The problem was that NameKind of a name used as a variable was not properly updated to KNameLocal, which later caused the identifier to be converted into IdenFunction by substitutionE called by the code handling named arguments. The function substitutionE calls toExpression on the name instead of on the identifier expression, and toExpression called on a name chooses the kind of identifier to create based on NameKind. In my opinion, it is error-prone to have two distinct places which determine the kind of an identifier. In genPatternDefs, I changed IdenFunction to IdenVar but forgot about adjusting NameKind.