<bit> exists in C++20 and is properly both constexpr and optimized. But
in C++17 mode, we don't have constexpr bitops and instead elect to have
performance at runtime instead. But somewhere along the line, either
when they were added, when C++20 <bit> support was, or in any of the
bugfixes for other compilers, the nesting of #ifdef got messed up and we
declared that we had constexpr builtins for MSVC in C++17 too.
The macro QT_HAS_CONSTEXPR_BUILTINS isn't supposed to be used by anyone
else... but we ended up not being able to use it ourselves either. So
I'm renaming it to a more precise label.
Pick-to: 6.5
Change-Id: I9671dee8ceb64aa9b9cafffd1741b9b4060c9753
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
e69d80e14d