appveyor.yml: Downgrade llvm and clang packages for the moment
appveyor.yml: Downgrade llvm and clang packages for the moment

The latest llvm and clang packages somehow break the doxygen binary. To fix
this use an older known to work version for now.

Signed-off-by: Alexandra Trifan <951af57c14d42c73475635752bd213fd996792b2@analog.com>
Signed-off-by: Lars-Peter Clausen <3318dc5ce3e4fb7c28a0b841b6801c884e1d0896@metafoo.de>

analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio