[regex] New enum type syntax_option_type

[Daniel Krügler]

Due to Tim Shen's recent activities at regex improvements we have in
libstdc++-v3/include/bits/regex_constants.h now

enum syntax_option_type { .. };

instead of the previous

typedef unsigned int syntax_option_type;

That change makes sense to me, but the question arises to me:
Shouldn't this enum get an explicit underlying type, such as

enum syntax_option_type : unsigned int { .. };

to keep the ABI more stable even if new enumerators would be added?

- Daniel