[Clang/CodeGen] Prevent crash if destructor class is not accessible (Part two)

[davide]

Summary

Phab doesn't allow me to reopen closed review or upload a new diff to a closed review. Anyhow.
This changes the code as Richard proposed. Thank you both for the time spent on this.

[davide]

For the archives, See D10508 for the whole story.

[rjmccall]

LGTM.

lib/CodeGen/CGClass.cpp

 HasTrivialDestructorBody(ASTContext &Context,
                          const CXXRecordDecl *BaseClassDecl,
                          const CXXRecordDecl *MostDerivedClassDecl)
 {
   // If the destructor is trivial we don't have to check anything else.
   if (BaseClassDecl->hasTrivialDestructor())
     return true;
 
-  // Give up if the destructor is not accessible.
-  if (!BaseClassDecl->getDestructor())
-    return false;
-
   if (!BaseClassDecl->getDestructor()->hasTrivialBody())
     return false;
 
   // Check fields.
   for (const auto *Field : BaseClassDecl->fields())
     if (!FieldHasTrivialDestructorBody(Context, Field))
       return false;
 

 {
   QualType FieldBaseElementType = Context.getBaseElementType(Field->getType());
 
   const RecordType *RT = FieldBaseElementType->getAs<RecordType>();
   if (!RT)
     return true;
 
   CXXRecordDecl *FieldClassDecl = cast<CXXRecordDecl>(RT->getDecl());
+
+  // The destructor for an implicit anonymous union member is never invoked.
+  if (FieldClassDecl->isUnion() && FieldClassDecl->isAnonymousStructOrUnion())
+    return false;
+
   return HasTrivialDestructorBody(Context, FieldClassDecl, FieldClassDecl);
 }
 
 /// CanSkipVTablePointerInitialization - Check whether we need to initialize
 /// any vtable pointers before calling this destructor.
 static bool CanSkipVTablePointerInitialization(ASTContext &Context,
                                                const CXXDestructorDecl *Dtor) {
   if (!Dtor->hasTrivialBody())