[tuple] Fix the return type of get().

For a tuple of references get() should still return a non-const reference,
even if the tuple itself is const:

  int n = 0;
  const tuple<int&> t(n);
  get<0>(t) = 1;  // post: n == 1

Without this property, tie() would be largely meaningless.

[[Imported from SVN: r6315]]