Implemented as an alias to keep c++2a working and remain consistent with
the way this was handled for the c++20 switch in cmake and the c++17
switch in qmake.
Pick-to: 6.2 6.3
Change-Id: I9a5f6d6b3a3adec748cf7207ceaa6da98d053cdb
Reviewed-by: Jörg Bornemann <joerg.bornemann@qt.io>