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

[Jonathan Wakely]

On 09/11/19 02:07 +0000, Smith-Rowland, Edward M wrote:
>Here is the <tuple> part of C++20 p1032 Misc constexpr bits.
>
>Tested on x86_64-linux. OK?

OK for trunk, thanks.