[sv-bc] return type of $bits

What is the return type of the $bits system function?  The LRM doesn't say.
I assume that it is either int or unsigned int.  Since it can never be
negative, it might make sense for it to be unsigned.  However, anyone
using -$bits might be surprised when it came out a large positive value
instead of the negative value they expected.

Steven Sharp
sharp@cadence.com