Introduce IR bit TYPE_INCLUDES_FLEXARRAY for the GCC extension

Introduce IR bit TYPE_INCLUDES_FLEXARRAY for the GCC extension

on a structure with a C99 flexible array member being nested in
another structure

GCC extension accepts the case when a struct with a flexible array member
is embedded into another struct or union (possibly recursively) as the last
field.
This patch is to introduce the IR bit TYPE_INCLUDES_FLEXARRAY (reuse the
existing IR bit TYPE_NO_NAMED_ARGS_SATDARG_P), set it correctly in C FE,
stream it correctly in Middle-end, and print it during IR dumping.

gcc/c/ChangeLog:

	* c-decl.cc (finish_struct): Set TYPE_INCLUDES_FLEXARRAY for
	struct/union type.

gcc/lto/ChangeLog:

	* lto-common.cc (compare_tree_sccs_1): Compare bit
	TYPE_NO_NAMED_ARGS_STDARG_P or TYPE_INCLUDES_FLEXARRAY properly
	for its corresponding type.

gcc/ChangeLog:

	* print-tree.cc (print_node): Print new bit type_include_flexarray.
	* tree-core.h (struct tree_type_common): Use bit no_named_args_stdarg_p
	as type_include_flexarray for RECORD_TYPE or UNION_TYPE.
	* tree-streamer-in.cc (unpack_ts_type_common_value_fields): Stream
	in bit no_named_args_stdarg_p properly for its corresponding type.
	* tree-streamer-out.cc (pack_ts_type_common_value_fields): Stream
	out bit no_named_args_stdarg_p properly for its corresponding type.
	* tree.h (TYPE_INCLUDES_FLEXARRAY): New macro TYPE_INCLUDES_FLEXARRAY.