[libc++][NFC] Replace _LIBCPP_INLINE_VISIBILTIY and _VSTD in <string>

Replace all the instances of `_LIBCPP_INLINE_VISIBILITY` with `_LIBCPP_HIDE_FROM_ABI` and `_VSTD` with `std`.

Reviewed By: Mordante, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D124662