[libcxx] Base MSVC autolinking on _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS

Previously the decision of which library to try to autolink was
based on _DLL, however the _DLL define (which is set by the compiler)
is tied to whether using a dynamically linked CRT or not, and the choice
of dynamic or static CRT is entirely orthogonal to whether libc++ is
linked dynamically or statically.

If _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS isn't defined, then all
declarations are decorated with dllimport, and there's no doubt that
the DLL version of the library is what must be linked.

_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS is defined if building with
LIBCXX_ENABLE_SHARED disabled, and thus the static library is what
should be linked.

If defining _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS manually but wanting
to link against the DLL version of the library, that's not a canonical
configuration, and then it's probably reasonable to manually define
_LIBCPP_NO_AUTO_LINK too, and manually link against the desired
library.

This fixes, among other issues, running tests for the library if
built with LIBCXX_ENABLE_STATIC disabled.

Differential Revision: https://reviews.llvm.org/D100539