Save discriminator info for LTO

Save discriminator info for LTO

for  gcc/ChangeLog

	* gimple-streamer-in.c (input_bb): Restore BB discriminator.
	* gimple-streamer-out.c (output_bb): Save it.
	* lto-streamer-in.c (input_struct_function_base): Restore
	instance discriminator if available.  Create map on demand.
	* lto-streamer-out.c (output_struct_function_base): Save it if
	available.
	* final.c (decl_to_instance_map): Document LTO strategy.

From-SVN: r263183