[OPENMP]Fix PR47158, case 2: do not report host-only functions in unused function in device mode.

If the function is not marked exlicitly as declare target and it calls
function(s), marked as declare target device_type(host), these host-only
functions should not be dignosed as used in device mode, if the caller
function is not used in device mode too.

Differential Revision: https://reviews.llvm.org/D86164