flatbuffers.h: fix C++11 compilation (#8857) (#8858)

This constexpr officially works only with C++14 (assignment to lhs, 2 statements) and actually broke some C++11 compilers

Author: greenrobot

include/flatbuffers/flatbuffers.h

@@ -260,15 +260,15 @@ inline const char* flatbuffers_version_string() {
     inline FLATBUFFERS_CONSTEXPR_CPP11 E operator ~ (E lhs){\
         return E(~T(lhs));\
     }\
-    inline FLATBUFFERS_CONSTEXPR_CPP11 E operator |= (E &lhs, E rhs){\
+    inline FLATBUFFERS_CONSTEXPR_CPP14 E operator |= (E &lhs, E rhs){\
         lhs = lhs | rhs;\
         return lhs;\
     }\
-    inline FLATBUFFERS_CONSTEXPR_CPP11 E operator &= (E &lhs, E rhs){\
+    inline FLATBUFFERS_CONSTEXPR_CPP14 E operator &= (E &lhs, E rhs){\
         lhs = lhs & rhs;\
         return lhs;\
     }\
-    inline FLATBUFFERS_CONSTEXPR_CPP11 E operator ^= (E &lhs, E rhs){\
+    inline FLATBUFFERS_CONSTEXPR_CPP14 E operator ^= (E &lhs, E rhs){\
         lhs = lhs ^ rhs;\
         return lhs;\
     }\