Fix ipa-modref selftest and destructor

Fix ipa-modref selftest and destructor

	* ipa-modref-tree.c: Add namespace selftest.
	(modref_tree_c_tests): Rename to ...
	(ipa_modref_tree_c_tests): ... this.
	* ipa-modref.c (pass_modref): Remove destructor.
	(ipa_modref_c_finalize): New function.
	* ipa-modref.h (ipa_modref_c_finalize): Declare.
	* selftest-run-tests.c (selftest::run_tests): Call
	ipa_modref_c_finalize.
	* selftest.h (ipa_modref_tree_c_tests): Declare.
	* toplev.c: Include ipa-modref-tree.h and ipa-modref.h
	(toplev::finalize): Call ipa_modref_c_finalize.