MINOR: debug: add a new DISGUISE() macro to pass a value as identity

MINOR: debug: add a new DISGUISE() macro to pass a value as identity

This does exactly the same as ALREADY_CHECKED() but does it inline,
returning an identical copy of the scalar variable without letting
the compiler know how it might have been transformed. This can
forcefully disable certain null-pointer checks or result checks when
known undesirable. Typically forcing a crash with *(DISGUISE(NULL))=0
will not cause a null-deref warning.