In the lowered IR there are often references to type parameters whose
containers are not in the current scope. This is incorrect semantically,
but it works in practice due to erasure, so when the mangler is used on
the lowered IR, we don't want to crash the compiler.
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