Bring back discontinued HashString from llvm 5.0
Bring back discontinued HashString from llvm 5.0

ashish-gehani/OCCAM,SRI-CSL/OCCAM,ashish-gehani/OCCAM,SRI-CSL/OCCAM,SRI-CSL/OCCAM,SRI-CSL/OCCAM,ashish-gehani/OCCAM,SRI-CSL/OCCAM,ashish-gehani/OCCAM