Move _PairT declaration out of __hash_combine to avoid warning under C++98

[dim]

Summary

Some parts of the FreeBSD tree are still compiled with C++98, and until
rL288554 this has always worked fine. After that, a complaint about the
newly introduced local _PairT is produced:

/usr/include/c++/v1/memory:3354:27: error: template argument uses local type '_PairT' [-Werror,-Wlocal-type-template-args]
    typedef __scalar_hash<_PairT> _HashT;
                          ^~~~~~
/usr/include/c++/v1/memory:3284:29: error: template argument uses local type '_PairT' [-Werror,-Wlocal-type-template-args]
    : public unary_function<_Tp, size_t>
                            ^~~
/usr/include/c++/v1/memory:3356:12: note: in instantiation of template class 'std::__1::__scalar_hash<_PairT, 2>' requested here
    return _HashT()(__p);
           ^

As far as I can see, there should be no problem moving the _PairT
struct to just before the __hash_combine() function, which fixes this
particular warning.

libcxx/trunk/include/memory

size_t operator()(_Tp __v) const _NOEXCEPT
                 size_t __d;
             } __s;
         } __u;
         __u.__t = __v;
         return __murmur2_or_cityhash<size_t>()(&__u, sizeof(__u));
     }
 };
 
-_LIBCPP_INLINE_VISIBILITY
-inline size_t __hash_combine(size_t __lhs, size_t __rhs) _NOEXCEPT {
-    struct _PairT {
-      size_t first;
-      size_t second;
-    };
+
+_LIBCPP_INLINE_VISIBILITY
+inline size_t __hash_combine(size_t __lhs, size_t __rhs) _NOEXCEPT {
     typedef __scalar_hash<_PairT> _HashT;
     const _PairT __p = {__lhs, __rhs};
     return _HashT()(__p);
 }
 
 template<class _Tp>
 struct _LIBCPP_TEMPLATE_VIS hash<_Tp*>
     : public unary_function<_Tp*, size_t>