Rewrite count(distinct if(bool, bitmap, null)) to bitmap_union_count

[liutang123]

Fix #4200

Add IF(BOOL, BITMAP, BITMAP) function.
Add UT.

[kangkaisen]

nullif, ifnull, coalesce function should also update.

[liutang123]

nullif, ifnull, coalesce function should also update.

Add ifnull(bitmap, bitmap) and coalesce(bitmap,...).

For nullif(bitmap, bitmap), I can not think of comparing two bitmaps.

[kangkaisen]

nullif, ifnull, coalesce function should also update.

Add ifnull(bitmap, bitmap) and coalesce(bitmap,...).

For nullif(bitmap, bitmap), I can not think of comparing two bitmaps.

OK. But PR not update.

[kangkaisen]

+1, LGTM