[Bug libstdc++/56158] bad enum values computed by operator~ in ios_base.h

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56158

--- Comment #13 from Jonathan Wakely <redi at gcc dot gnu.org> ---
N.B. we could also get rid of the _S_ios_xxx_end enumerators, but that would
break any code which (foolishly) refers to them, e.g. to suppress Clang's
-Wswitch warnings.

My suggestion assumes that __INT_MAX__ > (1 << 16), i.e. the compiler really
will choose int as the underlying type, but I think that's OK.