[libc++] Make everything in namespace std have default type visibility and...

[libc++] Make everything in namespace std have default type visibility and hidden visibility and remove _LIBCPP_ENUM_VIS

This avoids having to add `_LIBCPP_ENUM_VIS`, since that is handled through `type_visibility` and GCC always makes the visibility of enums default. It also fixes and missing `_LIBCPP_EXPORTED_FROM_ABI` on classes when using Clang.

Reviewed By: ldionne, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D153658