[libc++] Fix _LIBCPP_EXPORTED_FROM_ABI when visibility annotations are disabled
Fixes a bug where functions would get exported when building with
-fvisibility=hidden and defining _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS. No
visibility annotations should be added in this case.
The new logic for _LIBCPP_EXPORTED_FROM_ABI matches that of the other visibility
annotations around it.
Differential Revision: https://reviews.llvm.org/D55664