Function maybe_create_entry_wrapper 

pub fn maybe_create_entry_wrapper<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
    cx: &'a Bx::CodegenCx,
    cgu: &CodegenUnit<'tcx>,
) -> Option<Bx::Function>

Creates the main function which will initialize the rust runtime and call users main function.