* properly set global variables to const if they are not a global
   variable.
 * implement global variable initializations.
 * initial implementation of llvmType() for structs and functions.
 * implement genTypedValue for variable tags
 * implement more AIR instructions: varptr, slice_ptr, slice_len,
   slice_elem_val, ptr_slice_elem_val, unwrap_errunion_payload,
   unwrap_errunion_payload_ptr, unwrap_errunion_err,
   unwrap_errunion_err_ptr.