DWARF: late code range fixup

DWARF: late code range fixup

Ensure that the parent DWARF subprograms of offload kernel functions have a
code range, and are therefore not discarded by GDB.  This is only necessary
when the parent function does not actually exist in the final binary, which is
commonly the case within the offload device's binary.

This patch replaces 808bdf1bb29 and fdcb23540a2.  It should be squashed with
those before being posted upstream.

gcc/

	* gcc/dwarf2out.c (notional_parents_list): New file variable.
	(gen_subprogram_die): Record offload kernel functions in
	notional_parents_list.
	(fixup_notional_parents): New function.
	(dwarf2out_finish): Call fixup_notional_parents.
	(dwarf2out_c_finalize): Reset notional_parents_list.