[CIR] Add empty handlers for Using and UsingShadow decls (llvm#143032)

This adds emitTopLevelDecl "handlers" for Using and UsingShadow. These
don't actually need any handling, but they need to be present in the
switch to avoid hitting the default handler, which issues a diagnostic
about the decl kind not being implemented.

There are several other decl kinds that don't need any handling. Those
will be added when I have test cases for them.

Author: andykaylor

clang/lib/CIR/CodeGen/CIRGenModule.cpp

@@ -1135,6 +1135,7 @@ void CIRGenModule::emitTopLevelDecl(Decl *decl) {
     emitGlobalOpenACCDecl(cast<OpenACCDeclareDecl>(decl));
     break;
   case Decl::Enum:
+  case Decl::Using:          // using X; [C++]
   case Decl::UsingDirective: // using namespace X; [C++]
   case Decl::Typedef:
   case Decl::TypeAlias: // using foo = bar; [C++11]
@@ -1143,6 +1144,10 @@ void CIRGenModule::emitTopLevelDecl(Decl *decl) {
     assert(!cir::MissingFeatures::generateDebugInfo());
     break;
 
+  // No code generation needed.
+  case Decl::UsingShadow:
+    break;
+
   // C++ Decls
   case Decl::LinkageSpec:
   case Decl::Namespace:

clang/test/CIR/CodeGen/namespace.cpp

@@ -50,3 +50,46 @@ int f4(void) {
 // CHECK:   %[[G3_ADDR:.*]] = cir.get_global @_ZN4test5test22g3E : !cir.ptr<!s32i>
 // CHECK:   %[[G3_VAL:.*]] = cir.load{{.*}} %[[G3_ADDR]] : !cir.ptr<!s32i>, !s32i
 // CHECK:   %[[SUM2:.*]] = cir.binop(add, %[[SUM]], %[[G3_VAL]]) nsw : !s32i
+
+using test2::f3;
+using test2::g3;
+
+int f5() {
+  f3();
+  return g3;
+}
+
+// CHECK: cir.func @_Z2f5v()
+// CHECK:   cir.call @_ZN4test5test22f3Ev()
+// CHECK:   %[[G3_ADDR:.*]] = cir.get_global @_ZN4test5test22g3E : !cir.ptr<!s32i>
+// CHECK:   %[[G3_VAL:.*]] = cir.load{{.*}} %[[G3_ADDR]] : !cir.ptr<!s32i>, !s32i
+
+namespace test3 {
+  struct S {
+    int a;
+  } s;
+}
+
+using test3::s;
+
+int f6() {
+  return s.a;
+}
+
+// CHECK: cir.func @_Z2f6v()
+// CHECK:   cir.get_global @_ZN5test31sE : !cir.ptr<!rec_test33A3AS>
+// CHECK:   cir.get_member %{{.*}}[0] {name = "a"}
+
+int shadowedFunc() {
+  return 3;
+}
+
+namespace shadow {
+  using ::shadowedFunc;
+}
+
+void f7() {
+  shadow::shadowedFunc();
+}
+
+// CHECK: cir.func @_Z2f7v()