[cfe] Implement generic method instantiation on call of function object

[eernstg]

Thanks to @sgrekhov for raising this issue. As part of the constructor-tearoffs feature, it is no longer a compile-time error to perform a generic method instantiation on the call method of an expression whose static type is a generic function type. For instance:

X id<X>(X x) => x;

main() {
  var idVar = id;
  int Function(int) f = idVar.call;
  var g = idVar.call<int>;
}

The analyzer accepts this program as it should, but the CFE (dart from commit 02b2e6c) rejects it with the following error message:

n007.dart:5:31: Error: A value of type 'X Function<X>(X)' can't be assigned to a variable of type 'int Function(int)'.
  int Function(int) f = idVar.call;
                              ^

This issue is concerned with the implementation of support for doing this.