Previously, a function reference that used generic parameters from its
outer scope was lowered into a top-level non-generic subclass of
`FunctionN`, with `FunctionN` type arguments referencing type parameters
not present in the scope anymore. This sometimes resulted in generating
malformed mangled names.
From now on the generated subclass of `FunctionN` is generic. The needed
type arguments are passed upon instantiation, when the relevant generic
parameters are present in the scope.
Sergej Jaskiewicz