P2798 Fix layout mappings all static extent default constructor

[wg21bot]

P2798R0 Fix layout mappings all static extent default constructor (Christian Trott, Damien Lebrun-Grandie, Mark Hoemmen)

[billy-baker]

Presentation at March 28, 2023 telecon skipped based on feedback from Christian Trott.

The contents of P2798 were applied as LWG3876, which was approved at the Issaquah Feb 2023 meeting.