P1865 Add max() to latch and barrier

[wg21bot]

P1865R0 Add max() to latch and barrier (David Olsen, Olivier Giroux)

[dkolsen-pgi]

@FabioFracassi This paper is in support of NB comment US365 cplusplus/nbballot#361 . It should be considered for C++20 as part of the NB comment resolution.

[jensmaurer]

LEWG in Belfast: We disagree with the change to class template, the R0 of this paper is better (see notes). Forward P1865R0 to LWG for C++20.

[jensmaurer]

Reviewed by LWG in Belfast on Thu afternoon. Unanimous consent with 2 minor changes for c++20.