DebugInfo: Prefer vtable homing over ctor homing.

Vtables will be emitted in fewer places than ctors (every ctor
references the vtable, so at worst it's the same places - but at best
the type has a non-inline key function and the vtable is emitted in one
place)

Pulling this fix out of 517bbc64db which
was reverted in 4821508d4d

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -3351,7 +3351,7 @@ void CGDebugInfo::completeTemplateDefinition(
 }
 
 void CGDebugInfo::completeUnusedClass(const CXXRecordDecl &D) {
-  if (DebugKind <= codegenoptions::DebugLineTablesOnly)
+  if (DebugKind <= codegenoptions::DebugLineTablesOnly || D.isDynamicClass())
     return;
 
   completeClassData(&D);

clang/test/CodeGenCXX/debug-info-limited-ctor.cpp

@@ -77,3 +77,14 @@ void L() {
 // Check that types are being added to retained types list.
 // CHECK-DAG: !DICompileUnit{{.*}}retainedTypes: ![[RETAINED:[0-9]+]]
 // CHECK-DAG: ![[RETAINED]] = {{.*}}![[C]]
+
+
+struct VTableAndCtor {
+  virtual void f1();
+  VTableAndCtor();
+};
+
+VTableAndCtor::VTableAndCtor() {
+}
+
+// CHECK-DAG: !DICompositeType({{.*}}name: "VTableAndCtor", {{.*}}flags: DIFlagFwdDecl