lto-18 test

Demonstrates a reason to use IR symbols when deciding an --as-needed
library should be loaded.

	* testsuite/ld-plugin/lto-18a.c,
	* testsuite/ld-plugin/lto-18b.c,
	* testsuite/ld-plugin/lto-18c.c,
	* testsuite/ld-plugin/lto-18d.c,
	* testsuite/ld-plugin/lto-18.out: New test.
	* testsuite/ld-plugin/lto.exp: Run it.