* include/bits/alloc_traits.h (__cpp_lib_constexpr_dynamic_alloc):
	Define.
	(allocator_traits::_S_construct, allocator_traits::_S_destroy)
	(__alloc_on_copy, __alloc_on_move, __alloc_on_swap): Use
	_GLIBCXX14_CONSTEXPR instead of constexpr.
	* include/bits/stl_construct.h (_Destroy): Likewise.

From-SVN: r277588