Make __wrap_iter work with GCC again

Summary:
This bug was originally fixed in http://reviews.llvm.org/D7201. 

However it was broken again by the fix to https://llvm.org/bugs/show_bug.cgi?id=22605.

This patch re-fixes __wrap_iter with GCC by providing a forward declaration of <vector> before the friend declaration in __wrap_iter.
This patch avoids the issues in PR22605 by putting canonical forward declarations in <iosfwd> and including <iosfwd> in <vector>.

<iosfwd> was chosen as the canonical forward declaration headers for the following reasons:

1. `<iosfwd>` is small with almost no dependancies.
2. It already forward declares `std::allocator`
3. It is already included in `<iterator>` which we need to fix the GCC bug.

This patch fixes the test "gcc_workaround.pass.cpp"

Reviewers: mclow.lists, EricWF

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D16345

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@261382 91177308-0d34-0410-b5e6-96231b3b80d8

include/vector

@@ -262,6 +262,7 @@ void swap(vector<T,Allocator>& x, vector<T,Allocator>& y)
 */
 
 #include <__config>
+#include <iosfwd> // for forward declaration of vector
 #include <__bit_reference>
 #include <type_traits>
 #include <climits>
@@ -453,7 +454,7 @@ __vector_base<_Tp, _Allocator>::~__vector_base()
     }
 }
 
-template <class _Tp, class _Allocator = allocator<_Tp> >
+template <class _Tp, class _Allocator /* = allocator<_Tp> */>
 class _LIBCPP_TYPE_VIS_ONLY vector
     : private __vector_base<_Tp, _Allocator>
 {