All 105s had emergency HEAT rounds for self defense, much like M7 SPAGs and likewise. They usually were issued HEAT shells in sparse quantities; often numbered below 10 and were always in short supply, akin to HVAP or APCR in dedicated main battle tanks of the time period.

The long story short is: It wasn't. A seperate Tank Battalion usually had an "Assault Gun Platoon" of three (3) 105 Howitzers to provide anti-infantry support in a direct or near-direct fire role, provide smoke screens and reduce fortifications such as bunkers with their large-calibre HEAT and HE rounds. They are comparable to the M8 Self Propelled Howitzer in both role, numbers and employment. M8s were assigned to Mechanized Cavalry Squadrons and Groups (Separate) and provided the same close-support that 105s would do to for their heavier counterparts.

Any and all instances of 105 Howitzers (be they Preists, or Shermans proper) being used in an Anti-Tank role are usually by necessity, not design. I can think of no better example then when Artillery and HQ units of the 2nd AD and VII Corps were forced to form blocking positions to eradicate Germans of the 2.SS and 17.SS trying to break out of the Roncey and Coutances pockets, respectively. I repeat: Emergency use only.

Sources:

Houston, "Hell on Wheels: A History of the 2nd Armored"

Carafano, "Operation Cobra: Beyond D-Day"