[libcxx-dev] _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS on Windows

[Petr Hosek via libcxx-dev]

I'm setting up a Clang toolchain build on Windows. We need to include
runtimes for several targets including Windows, Linux and Fuchsia. In the
case of Windows and Linux we only want to provide static libc++. In the
case of Fuchsia we want to provide both static and shared.

This leads to an issue: because of this condition
https://github.com/llvm/llvm-project/blob/9a48411f35a122ac8755a8453887b919c77daf3f/libcxx/CMakeLists.txt#L861,
_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS
is set to ON for Windows, but to OFF on Linux and Fuchsia. Since this
definition is a part of __config_site.in
https://github.com/llvm/llvm-project/blob/9a48411f35a122ac8755a8453887b919c77daf3f/libcxx/include/__config_site.in#L28
we cannot use a single __config that would be usable for all targets which
breaks the build of our toolchain.

Does anyone know why we have that condition? Why is it only restricted to
Windows? It's not clear to me why libc++ on Windows should be different
from other platforms. Users that want to disable visibility annotations can
always set _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS in their build which is
what projects like Chromium and Fuchsia already do.
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