2019-11-16  Edward Smith-Rowland  <3dw4rd@verizon.net>

	Repair the <tuple> part of C++20 p1032 Misc constexpr bits.
	* include/bits/uses_allocator.h (__uses_alloc0::_Sink::operaror=)
	(__use_alloc(const _Alloc&)) : Constexpr.

From-SVN: r278373