Full type name qualification fails for function types / argument types

[Axel-Naumann]

The underlying bug of #8387 is the lack of full qualification of the argument types of function types: with

namespace N {
  struct S {};
  using Func_t = int(S*);
}

TClass::GetClass("vector<Func_t>") should fully qualify the name to "vector<int(N::S*)>" but instead leaves it at "vector<int(S*)>": cling/lib/Utils/AST.cpp's GetPartiallyDesugaredTypeImpl() checks whether the type is a TemplateSpecializationType and iterates over the template args, but it doesn't check whether it's a FunctionProtoType to iterate over the argument types.

That's a bug which might be relevant for cases like this:

template <class ACTION>
class ICanBeSerializedJustFine {
  ACTION fAction; ///<!
  int fSerializeThis;
};

Note: this relates (was discovered through) #8387