Fix violations of [basic.class.scope]p2.

These cases all follow the same pattern:

struct A {
  friend class X;
  //...
  class X {};
};

But 'friend class X;' injects 'X' into the surrounding namespace scope,
rather than introducing a class member. So the second 'class X {}' is a
completely different type, which changes the meaning of the earlier name
'X' from '::X' to 'A::X'.

Additionally, the friend declaration is pointless -- members of a class
don't need to be befriended to be able to access private members.