PR libstdc++/88840 delay evaluation of constant until type is complete

PR libstdc++/88840 delay evaluation of constant until type is complete

Clang fails to compile std::vector<Incomplete> because the static member
__use_relocate cannot be evaluated for an incomplete type. Replace with
a static member function that will not be odr-used until needed, by
which point the type must be complete.

	PR libstdc++/88840
	* include/bits/stl_vector.h (vector::__use_relocate): Replace static
	data member with static member function _S_use_relocate().
	* include/bits/vector.tcc (vector::reserve, vector::_M_realloc_insert)
	(vector::_M_default_append): Use _S_use_relocate() instead of
	__use_relocate.

From-SVN: r268239