Implement the <array> part of C++20 p1032 Misc constexpr bits.

[Jonathan Wakely]

On 09/11/19 01:15 +0000, Smith-Rowland, Edward M wrote:
>I'm going to implement p1032 in pieces.  It *is* miscellaneous after all ;-).
>
>Tested on x96_64-linux? OK?

>2019-11-09  Edward Smith-Rowland  <3dw4rd@verizon.net>
>
>	Implement the <array> part of C++20 p1032 Misc constexpr bits.
>	* include/std/array (fill, swap): Make constexpr.
>	* testsuite/23_containers/array/requirements/constexpr_fill.cc: New.
>	* testsuite/23_containers/array/requirements/constexpr_swap.cc: New.

OK for trunk, thanks.