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

llvm-svn: 261382