Return false from __has_declspec_attribute() if not explicitly enabled

Return false from __has_declspec_attribute() if not explicitly enabled

Currently, projects can check for __has_declspec_attribute() and use
it accordingly, but the check for __has_declspec_attribute will return
true even if declspec attributes are not enabled for the target.

This changes Clang to instead return false when declspec attributes are
not supported for the target.