It would be as safe as the water coursing through my copper pipes, joined with solder... probably the same 60Sn-40Pb composition solder.
Of course, some automotive heater cores may be something other than a soldered copper. They may be a newer design that uses solder with no lead or they may be made of aluminum.
probably made of aluminum on anything made recently, Cu is too expensive.
Even if it is soldered w/lead, as Kestas said it's of such a minor amount as to be inconsequential unless you are an environmental worrywart. But it's more likely a higher temp solder than 60/40, with even less lead and possibly some silver.
You are going to drink water that has circulated thru the heater core? What is heating the core? Is this merely an old coil that you now wish to use in a potable water system or a brand new coil to be adapted for this purpose? What does the coil do in the potable water system?