To improve code quality and to avoid surprise build failures on Windows, we should fix any -Wconversion warnings that are reported by gcc or clang.
gcc appears to be stricter than clang here; it will require explicit bitmask operations to suppress warnings about assignments to bit fields in structs.
As a first step, we should enable -Wconversion in InnoDB and Mariabackup. This will require some headers in include/ and sql/ to be fixed as well. The Mariabackup build also includes some compilation units in libmysqld.