Fix PR27684 - std::tuple no longer accepts reference to incomplete type in some cases.

Libc++ has to deduce the 'allocator_arg_t' parameter as 'AllocArgT' for the
following constructor:

  template <class Alloc> tuple(allocator_arg_t, Alloc const&)

Previously libc++ has tried to support tags derived from 'allocator_arg_t' by
using 'is_base_of<AllocArgT, allocator_arg_t>'. However this breaks whenever a
2-tuple contains a reference to an incomplete type as its first parameter.

See https://llvm.org/bugs/show_bug.cgi?id=27684

llvm-svn: 273334