re PR lto/47247 (Linker plugin specification makes it difficult to handle COMDATs)

	PR lto/47247
	* lto-plugin.c (get_symbols_v2): New variable.
	(write_resolution): Use V2 API when available.
	(onload): Handle LDPT_GET_SYMBOLS_V2.

	* lto-symtab.c (lto_symtab_resolve_symbols): Do not resolve
	when resolution is already availbale from plugin.
	(lto_symtab_merge_decls_1): Handle LDPR_PREVAILING_DEF_IRONLY_EXP.
	* cgraph.c (ld_plugin_symbol_resolution): Add prevailing_def_ironly_exp.
	* lto-cgraph.c (LDPR_NUM_KNOWN): Update.
	* ipa.c (varpool_externally_visible_p): IRONLY variables are never
	externally visible.
	* varasm.c (resolution_to_local_definition_p): Add
	LDPR_PREVAILING_DEF_IRONLY_EXP.
	(resolution_local_p): Likewise.

	* common.c (lto_resolution_str): Add new resolution.
	* common.h (lto_resolution_str): Likewise.

From-SVN: r179424