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

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.

Reviewers: mclow.lists, EricWF

Subscribers: cfe-commits, emaste

Differential Revision: https://reviews.llvm.org/D28472

llvm-svn: 291476