Avoid g++.dg/torture/pr106922.C FAIL with the pre-C++11 ABI

The following forces the g++.dg/torture/pr106922.C testcase to use
the C++11 libstdc++ ABI and checks whether that worked.

gcc/testsuite/
	* g++.dg/torture/pr106922.C: Force _GLIBCXX_USE_CXX11_ABI to 1.

gcc/testsuite/g++.dg/torture/pr106922.C

@@ -4,8 +4,16 @@
 // -O1 doesn't iterate VN and thus has bogus uninit diagnostics
 // { dg-skip-if "" { *-*-* } { "-O1" } { "" } }
 
+// The testcase still emits bogus diagnostics with the pre-C++11 ABI
+#undef _GLIBCXX_USE_CXX11_ABI
+#define _GLIBCXX_USE_CXX11_ABI 1
+
 #include <vector>
 
+// When the library is not dual-ABI and defaults to old just compile
+// an empty TU
+#if _GLIBCXX_USE_CXX11_ABI
+
 #include <optional>
 template <class T>
 using Optional = std::optional<T>;
@@ -46,3 +54,4 @@ void test()
         externals.external2 = internal2;
     }
 }
+#endif