Question

it has (rain) all night​

Answer 1

Here, the correct use will be the first one, ie, It is raining heavily. This is a complete sentence itself. You can use the second one in informal/casual conversation. It is more of a cropped form of : It is now raining heavier than it was earlier..

Answer 2

mate here we go with your answer...

the correct verb form to be used in this question is.. Has been raining.

this implies that the rain is continuing all the night.

hope it helps you..