Use the IR symbol table for the IR input object

Use the IR symbol table for the IR input object

ELF linker shouldn't skip the IR object when searching the symbol table
of an archive element.  If linker doesn't know if the object file is an
IR object, it should give LTO plugin a chance to get the correct symbol
table and use the IR symbol table if the input is an IR object.

bfd/

	PR ld/18250
	PR ld/20267
	* elflink.c: Include plugin.h if BFD_SUPPORTS_PLUGINS is
	defined.
	(elf_link_is_defined_archive_symbol): Call
	bfd_link_plugin_object_p on unknown plugin object and use the
	IR symbol table if the input is an IR object.
	* plugin.c (bfd_link_plugin_object_p): New function.
	* plugin.h (bfd_link_plugin_object_p): New prototype.

ld/

	PR ld/20267
	* testsuite/ld-plugin/lto.exp (lto_link_tests): Add test for
	PR ld/20267.
	(lto_run_tests): Likewise.
	* testsuite/ld-plugin/pr20267a.c: New file.
	* testsuite/ld-plugin/pr20267b.c: Likewise.