Don't emit an available_externally vtable pointing to linkonce_odr funcs.

This fixes pr13124.

From the discussion at
http://lists.cs.uiuc.edu/pipermail/cfe-dev/2012-June/022606.html
we know that we cannot make funcions in a weak_odr vtable also weak_odr. They
should remain linkonce_odr.

The side effect is that we cannot emit a available_externally vtable unless we
also emit a copy of the function. This also has an issue: If codegen is going
to output a function, sema has to mark it used. Given llvm.org/pr9114, it looks
like sema cannot be more aggressive at marking functions used because
of vtables.

This leaves us with a few unpleasant options:

* Marking functions in vtables used if possible. This sounds a bit sloppy, so
  we should avoid it.
* Producing available_externally vtables only when all the functions in it are
  already used or weak_odr. This would cover cases like

--------------------
struct foo {
  virtual ~foo();
};
struct bar : public foo {
  virtual void zed();
};
void f() {
  foo *x(new bar);
  delete x;
}
void g(bar *x) {
  x->~bar(); // force the destructor to be used
}
--------------------------

and

----------------------------------
template<typename T>
struct bar {
  virtual ~bar();
};
template<typename T>
bar<T>::~bar() {
}

// make the destructor weak_odr instead of linkonce_odr
extern template class bar<int>;

void f() {
  bar<int> *x(new bar<int>);
  delete x;
}
----------------------------

These look like corner cases, so it is unclear if it is worth it.

* And finally: Just nuke this optimization. That is what this patch implements.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189852 91177308-0d34-0410-b5e6-96231b3b80d8