This patch removes most of the checks that are made using C++20
__cpp_* macros for features available in C++17 and earlier.
Library feature check macros (__cpp_lib_*) are unaffected.
Change-Id: I557b2bd0d4ff09b13837555e9880eb28e0355f64
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>