Solve constraints in unary_op_arg_type/2

This is here to track:

Lines 3233 to 3236 in 1498d17

    
           %% TODO: this should be more specific once we're able to solve constraints on type vars. 
        
           %%       Otherwise, we'll lose some feedback / precision. 
        
           %%       With constraint solving, if the type variable resolves to pos_integer() 
        
           %%       and _Op == '-' here, then we should have returned neg_integer().

	%% TODO: this should be more specific once we're able to solve constraints on type vars.
	%% Otherwise, we'll lose some feedback / precision.
	%% With constraint solving, if the type variable resolves to pos_integer()
	%% and _Op == '-' here, then we should have returned neg_integer().