[libc++] Improve error messages for disabled modes

We should not surface CMake-level options like LIBCXX_ENABLE_FILESYSTEM
to our users, since they don't know what it means. Instead, use a slightly
more general wording.

Also, add an error in <ios> to improve the quality of errors for people
trying to use <iostream> when localization is disabled.

Differential Revision: https://reviews.llvm.org/D125910