Amend class template instantiation test for bug 207840

Enabling C++17 deduction guides unconditionally causes one of the tests for
template instantiation to fail because one of instantiations can be done via
implicit deduction guide using default template arguments.
Test case is covering issue https://bugs.eclipse.org/bugs/show_bug.cgi?id=207840
and remaining erroneous cases are not affected.

Amend the test and comment about change since C++17.

core/org.eclipse.cdt.core.tests/parser/org/eclipse/cdt/core/parser/tests/ast2/AST2TemplateTests.java

@@ -3096,14 +3096,14 @@ public void testNonAmbiguityCase_229942() throws Exception {
 	//	B b1;
 	//	B<> b2; // error - no default args
 	//
-	//	C c1;
+	//	C c1;   // OK since C++17 via implicit deduction guide using default template argument
 	//	C<> c2; // ok - default args
 	public void testMissingTemplateArgumentLists() throws Exception {
 		BindingAssertionHelper ba = new AST2AssertionHelper(getAboveComment(), CPP);
 		ba.assertProblem("B b1", 1);
 		ba.assertNonProblem("B<> b2", 1, ICPPTemplateDefinition.class, ICPPClassType.class);
 		ba.assertProblem("B<> b2", 3);
-		ba.assertProblem("C c1", 1);
+		ba.assertNonProblem("C c1", 1); // OK since C++17
 		ba.assertNonProblem("C<> c2", 1, ICPPTemplateDefinition.class, ICPPClassType.class);
 		ba.assertNonProblem("C<> c2", 3, ICPPTemplateInstance.class, ICPPClassType.class);
 	}