Make __wrap_iter work with GCC again

[EricWF]

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"

[EricWF]

@mclow.lists said this was OK to commit if he couldn't think of a better idea. It's unfortunate to use <iosfwd> to forward declare containers but <vector> already included <iosfwd> indirectly so theres no extra cost to putting the forward declaration there.

include/iosfwd

 
 template <class _CharT,             // for <stdexcept>
           class _Traits = char_traits<_CharT>,
           class _Allocator = allocator<_CharT> >
     class _LIBCPP_TYPE_VIS_ONLY basic_string;
 typedef basic_string<char, char_traits<char>, allocator<char> > string;
 typedef basic_string<wchar_t, char_traits<wchar_t>, allocator<wchar_t> > wstring;
 
+
+// Include other forward declarations here
+template <class _Tp, class _Alloc = allocator<_Tp> >
+class _LIBCPP_TYPE_VIS_ONLY vector;
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif  // _LIBCPP_IOSFWD

include/iterator

 template <class T, size_t N> constexpr T* data(T (&array)[N]) noexcept;           // C++17
 template <class E> constexpr const E* data(initializer_list<E> il) noexcept;      // C++17
 
 }  // std
 
 */
 
 #include <__config>
+#include <iosfwd> // for forward declarations of vector and string.
 #include <__functional_base>
 #include <type_traits>
 #include <cstddef>
-#include <iosfwd>
 #include <initializer_list>
 #ifdef __APPLE__
 #include <Availability.h>
 #endif
 
 #include <__debug>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)

include/vector

 void swap(vector<T,Allocator>& x, vector<T,Allocator>& y)
     noexcept(noexcept(x.swap(y)));
 
 }  // std
 
 */
 
 #include <__config>
+#include <iosfwd> // for forward declaration of vector
 #include <__bit_reference>
 #include <type_traits>
 #include <climits>
 #include <limits>
 #include <initializer_list>
 #include <memory>
 #include <stdexcept>
 #include <algorithm>

 {
     if (__begin_ != nullptr)
     {
         clear();
         __alloc_traits::deallocate(__alloc(), __begin_, capacity());
     }
 }
 
-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>
 {
 private:
     typedef __vector_base<_Tp, _Allocator>           __base;
     typedef allocator<_Tp>                           __default_allocator_type;
 public:
     typedef vector                                   __self;