lto: LTO cgraph support for late declare variant resolution [PR96680]

lto: LTO cgraph support for late declare variant resolution [PR96680]

> I've tried to add the saving/restoring next to ipa refs saving/restoring, as
> the declare variant alt stuff is kind of extension of those, unfortunately
> following doesn't compile, because I need to also write or read a tree there
> (ctx is a portion of DECL_ATTRIBUTES of the base function), but the ipa refs
> write/read back functions don't have arguments that can be used for that.

This patch adds the streaming out and in of those omp_declare_variant_alt
hash table on the side data for the declare_variant_alt cgraph_nodes and
treats for LTO purposes the declare_variant_alt nodes (which have no body)
as if they contained a body that calls all the possible variants.
After IPA all the calls to these magic declare_variant_alt calls are
replaced with call to one of the variant depending on which one has the
highest score in the context.

2020-10-28  Jakub Jelinek  <jakub@redhat.com>

	PR lto/96680
gcc/
	* lto-streamer.h (omp_lto_output_declare_variant_alt,
	omp_lto_input_declare_variant_alt): Declare variant.
	* symtab.c (symtab_node::get_partitioning_class): Return
	SYMBOL_DUPLICATE for declare_variant_alt nodes.
	* passes.c (ipa_write_summaries): Add declare_variant_alt to
	partition.
	* lto-cgraph.c (output_refs): Call omp_lto_output_declare_variant_alt
	on declare_variant_alt nodes.
	(input_refs): Call omp_lto_input_declare_variant_alt on
	declare_variant_alt nodes.
	* lto-streamer-out.c (output_function): Don't call
	collect_block_tree_leafs if DECL_INITIAL is error_mark_node.
	(lto_output): Call output_function even for declare_variant_alt
	nodes.
	* omp-general.c (omp_lto_output_declare_variant_alt,
	omp_lto_input_declare_variant_alt): New functions.
gcc/lto/
	* lto-common.c (lto_fixup_prevailing_decls): Don't use
	LTO_NO_PREVAIL on TREE_LIST's TREE_PURPOSE.
	* lto-partition.c (lto_balanced_map): Treat declare_variant_alt
	nodes like definitions.
libgomp/
	* testsuite/libgomp.c/declare-variant-1.c: New test.