Added _eval_Abs for polar_lift

Abs is the same on every branch, so this is ok.
fixes #7450
```
>>> Abs(polar_lift(-1-I))
sqrt(2)
>>> integrate(exp(-(1+I)*x), (x, 0, oo))
sqrt(2)*exp(-I*pi/4)/2
```
Added a test for Abs(polar_lift(..))