位域的符号Sign of Bit Fields
int 域被视为 signed int 位域还是 unsigned int 位域ANSI 188.8.131.52 Whether a "plain"
int field is treated as a signed int bit field or as an unsigned int bit field
位域可以带符号也可以不带符号。Bit fields can be signed or unsigned. 纯位域被视为带符号的。Plain bit fields are treated as signed.