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


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@291476 91177308-0d34-0410-b5e6-96231b3b80d8

include/memory

@@ -3344,12 +3344,13 @@ struct __scalar_hash<_Tp, 4>
     }
 };
 
+struct _PairT {
+  size_t first;
+  size_t second;
+};
+
 _LIBCPP_INLINE_VISIBILITY
 inline size_t __hash_combine(size_t __lhs, size_t __rhs) _NOEXCEPT {
-    struct _PairT {
-      size_t first;
-      size_t second;
-    };
     typedef __scalar_hash<_PairT> _HashT;
     const _PairT __p = {__lhs, __rhs};
     return _HashT()(__p);