Function LLVMGetPrefixData 

pub unsafe extern "C" fn LLVMGetPrefixData(
    Fn: LLVMValueRef,
) -> LLVMValueRef

Gets the prefix data associated with a function. Only valid on functions, and only if LLVMHasPrefixData returns true. See https://llvm.org/docs/LangRef.html#prefix-data