include/winnt: Fix constexpr-ness of compound assignment operators of enum types

Previously they cast their first enum operands to `int&` and operate on
integers, which is type-punning and maybe not safe. This commit makes them
call the corresponding non-assignment operators instead. As they modify their
arguments, they are only `constexpr` since C++14.

Also regroup them a little.

Signed-off-by: LIU Hao <lh_mouse@126.com>