absolute: track the absolute limits that variables can be

absolute: track the absolute limits that variables can be

If you have an assignment like this "int x = (unsigned char)y;" then you
know that x is a number between 0-255.  This is to track those kinds of
things.

smatch_extra.c tracks some of these things right now, but it's not really
the right thing.  Smatch extra should be tracking stronger implications.
Tracking this stuff in Smatch extra leads to buffer overflow false
positives.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>