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>

mingw-w64-headers/include/winnt.h

diff --git a/mingw-w64-headers/include/winnt.h b/mingw-w64-headers/include/winnt.h
index 007539a..27e02de 100644
--- a/mingw-w64-headers/include/winnt.h
+++ b/mingw-w64-headers/include/winnt.h
@@ -706,13 +706,13 @@
 #ifdef __cplusplus
 #define DEFINE_ENUM_FLAG_OPERATORS(ENUMTYPE) \
 extern "C++" { \
-__MINGW_CXX11_CONSTEXPR inline ENUMTYPE operator | (ENUMTYPE a, ENUMTYPE b) { return ENUMTYPE(((int)a) | ((int)b)); } \
-__MINGW_CXX11_CONSTEXPR inline ENUMTYPE &operator |= (ENUMTYPE &a, ENUMTYPE b) { return (ENUMTYPE &)(((int &)a) |= ((int)b)); } \
-__MINGW_CXX11_CONSTEXPR inline ENUMTYPE operator & (ENUMTYPE a, ENUMTYPE b) { return ENUMTYPE(((int)a) & ((int)b)); } \
-__MINGW_CXX11_CONSTEXPR inline ENUMTYPE &operator &= (ENUMTYPE &a, ENUMTYPE b) { return (ENUMTYPE &)(((int &)a) &= ((int)b)); } \
 __MINGW_CXX11_CONSTEXPR inline ENUMTYPE operator ~ (ENUMTYPE a) { return ENUMTYPE(~((int)a)); } \
+__MINGW_CXX11_CONSTEXPR inline ENUMTYPE operator | (ENUMTYPE a, ENUMTYPE b) { return ENUMTYPE(((int)a) | ((int)b)); } \
+__MINGW_CXX11_CONSTEXPR inline ENUMTYPE operator & (ENUMTYPE a, ENUMTYPE b) { return ENUMTYPE(((int)a) & ((int)b)); } \
 __MINGW_CXX11_CONSTEXPR inline ENUMTYPE operator ^ (ENUMTYPE a, ENUMTYPE b) { return ENUMTYPE(((int)a) ^ ((int)b)); } \
-__MINGW_CXX11_CONSTEXPR inline ENUMTYPE &operator ^= (ENUMTYPE &a, ENUMTYPE b) { return (ENUMTYPE &)(((int &)a) ^= ((int)b)); } \
+__MINGW_CXX14_CONSTEXPR inline ENUMTYPE& operator |= (ENUMTYPE& a, ENUMTYPE b) { return a = a | b; } \
+__MINGW_CXX14_CONSTEXPR inline ENUMTYPE& operator &= (ENUMTYPE& a, ENUMTYPE b) { return a = a & b; } \
+__MINGW_CXX14_CONSTEXPR inline ENUMTYPE& operator ^= (ENUMTYPE& a, ENUMTYPE b) { return a = a ^ b; } \
 }
 #else
 #define DEFINE_ENUM_FLAG_OPERATORS(ENUMTYPE) /* */