x/vuln/vulncheck: support for generics

[zpavlinovic]

vulncheck currently supports generics by instantiating them using ssa.InstantiateGenerics. But what happens for entry points that are generics functions: they themselves are not instantiated. They should in principle be supported just as functions accepting interfaces arguments, but it could be that the underlying vta callgraph is not handling them properly.

Investigate and make a long-term decision.

[zpavlinovic]

Entry points that are generics are not currently computed in the set of entry points. ssa.MethodValue returns nil for such methods and functions.