[libcxx] Make __wrap_iter work with gcc.

he following snippet doesn't build when using gcc and libc++:

    #include <string>
    void f(const std::string& s) { s.begin(); }
    #include <vector>
    void AppendTo(const std::vector<char>& v) { v.begin(); }

The problem is that __wrap_iter has a private constructor. It lists vector<>
and basic_string<> as friends, but gcc seems to ignore this for vector<> for
some reason. Declaring vector before the friend declaration in __wrap_iter is
enough to work around this problem, so do that. With this patch, I'm able to
build chromium/android with libc++. Without it, two translation units fail to
build. (iosfwd already provides a forward declaration of basic_string.)

As far as I can tell, this is due to a gcc bug, which I filed as
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64816.

Fixes PR22355.

http://reviews.llvm.org/D7201

llvm-svn: 227226