[2021-02 LWG Motion 7] P1682R2 std::to_underlying for enumeration

[jensmaurer]

P1682R2

[jensmaurer]

Successor: P1682R3

[jwakely]

Successor: P1682R3

For the record, the LWG motion should have been for the R3 revision, but I screwed up. But the only differences between R2 and R3 are editorial anyway. The changes merged by #4526 incorporate those editorial fixes.