What are my options for replacing a rotted fence post that is slanted in a large concrete slab?
How many of these posts do you want to replace? Where I live most people are now using round tubular galvanized steel posts about 2-3/8" OD instead of 4x4" cedar or pressure treated. There are special brackets for the rails that clamp around the poles. These poles do come in different grades (wall thicknesses) and in your case I would think a better grade would be appropriate since removal from concrete in these 4x4" holes would be very difficult compared to poles in holes in the ground even set in concrete. 

If you put those galvanized round poles in the 4x4 holes, there would be enough play to get them plumb and then pour in concrete. There is also a kind of polymer foam for posts which is touted to be secure, but more workable than concrete. https://www.homedepot.com/p/Sika-33-fl-oz-Fence-Post-Mix-483503/205671519