testsuite: Fix warning regression due to std::string changes [PR106795]

std::string now has [[nodiscard]] attributes on most members, causing
-Wunused-result warnings for this test.

gcc/testsuite/ChangeLog:

	PR testsuite/106795
	* g++.dg/tree-ssa/empty-loop.C: Use -Wno-unused-result.

Author: jwakely

gcc/testsuite/g++.dg/tree-ssa/empty-loop.C

@@ -1,5 +1,5 @@
 /* { dg-do compile } */
-/* { dg-options "-O2 -fdump-tree-cddce2 -ffinite-loops" } */
+/* { dg-options "-O2 -fdump-tree-cddce2 -ffinite-loops -Wno-unused-result" } */
 
 #include <string>
 #include <vector>