[PATCH, libstdc++] Implement C++20 p1032 default_searcher constexprosity.

[Ville Voutilainen]

On Fri, 15 Nov 2019 at 22:16, Smith-Rowland, Edward M
<ESmith-rowland@alionscience.com> wrote:
>
> Pretty self-explanatory.

LGTM. Jonathan still needs to ack it.