[PATCH] rewrite type_difference()

[PATCH] rewrite type_difference()

a) qualifiers should not be ignored beyond the top layer
b) qualifiers *should* be ignored in top layer of function arguments
c) change prototype - pass ctype * instead of symbol * and pass
"what to add to modifiers in that ctype" instead of "what to ignore".

We are still not quite done (there are incomplete types, there
are array size comparisons, there is lifting of signedness logics
into compatible_assignment_types()), but it's a much better approximation.

BTW, it's already good enough for kernel ARRAY_SIZE(), which
has become a major source of noise lately...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
[Josh: prototype fix]
Signed-off-by: Josh Triplett <josh@freedesktop.org>