[libc++] Add _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS

It's useful to be able to disable visibility annotations entirely; for
example, if we're building libc++ static to include in another library,
and we don't want any libc++ functions getting exported out of that
library. This is a generalization of _LIBCPP_DISABLE_DLL_IMPORT_EXPORT.

Differential Revision: https://reviews.llvm.org/D26934

llvm-svn: 288690