[Bug libstdc++/67996] std::ios_base::seekdir raises -Wswitch with Clang

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67996

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Kim Gräsman from comment #0)
> What's the motivation for _S_ios_seekdir_end? Any chance it could be removed?

Presumably to make sure the enum type is more than 16-bits, which could be done
with a fixed underlying type in C++11, but not in C++03.

I don't think we should change this just because of a warning. The std::lib is
allowed to define whatever enumerators it wants to (as long as they use
reserved names).