An implementation of C++20 std::to_address, a prerequesite for
QSpan<>.
The test cases are inspired by libstdc++'s test suite, just to avoid
missing some cases, but the to_address implementation is white-room.
Fixes: QTBUG-108430
Change-Id: I4c092fdd7a56c0b279068e341bbf91a725ca3b1f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
