[CIR][CIRGen] Add missing testcase for null base class

Forgot to git add in cb0cb34

Author: bcardosolopes

clang/test/CIR/CodeGen/const-baseclass.cpp

@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-cir %s -o %t.cir
+// RUN: FileCheck --input-file=%t.cir %s
+
+struct Empty { };
+struct A { 
+};
+
+struct B : A, Empty { 
+  B() : A(), Empty() { }
+};
+
+void f() {
+  B b1;
+}
+
+// CHECK-LABEL: @_ZN1BC2Ev
+// CHECK: %[[A:.*]] = cir.base_class_addr({{.*}}) [0] -> !cir.ptr<!ty_A>
+// CHECK: cir.call @_ZN1AC2Ev(%[[A:.*]]) : (!cir.ptr<!ty_A>) -> ()
+// CHECK: %[[BASE:.*]] = cir.base_class_addr({{.*}}) [0] -> !cir.ptr<!ty_Empty>
+// CHECK: cir.call @_ZN5EmptyC2Ev(%[[BASE]]) : (!cir.ptr<!ty_Empty>) -> ()