Satisfy GCC class-memaccess warnings

GCC 8 added a -Wclass-memaccess that is enabled in -Wall that warns when
making direct memory modifications to non-trivial objects. The objects
that are modified here are only "non-trivial" in non-interesting ways
(e.g., removal of copy constructor) and can be safely modified as is
being done.

This simply explicitly casts the pointers before calling memset to
indicate that they should be treated as raw memory.

src/google/protobuf/descriptor.cc

@@ -3860,7 +3860,7 @@ Symbol DescriptorPool::NewPlaceholderWithMutexHeld(
     placeholder_file->enum_types_ = tables_->AllocateArray<EnumDescriptor>(1);
 
     EnumDescriptor* placeholder_enum = &placeholder_file->enum_types_[0];
-    memset(placeholder_enum, 0, sizeof(*placeholder_enum));
+    memset(static_cast<void*>(placeholder_enum), 0, sizeof(*placeholder_enum));
 
     placeholder_enum->full_name_ = placeholder_full_name;
     placeholder_enum->name_ = placeholder_name;
@@ -3874,7 +3874,7 @@ Symbol DescriptorPool::NewPlaceholderWithMutexHeld(
     placeholder_enum->values_ = tables_->AllocateArray<EnumValueDescriptor>(1);
 
     EnumValueDescriptor* placeholder_value = &placeholder_enum->values_[0];
-    memset(placeholder_value, 0, sizeof(*placeholder_value));
+    memset(static_cast<void*>(placeholder_value), 0, sizeof(*placeholder_value));
 
     placeholder_value->name_ = tables_->AllocateString("PLACEHOLDER_VALUE");
     // Note that enum value names are siblings of their type, not children.
@@ -3894,7 +3894,7 @@ Symbol DescriptorPool::NewPlaceholderWithMutexHeld(
     placeholder_file->message_types_ = tables_->AllocateArray<Descriptor>(1);
 
     Descriptor* placeholder_message = &placeholder_file->message_types_[0];
-    memset(placeholder_message, 0, sizeof(*placeholder_message));
+    memset(static_cast<void*>(placeholder_message), 0, sizeof(*placeholder_message));
 
     placeholder_message->full_name_ = placeholder_full_name;
     placeholder_message->name_ = placeholder_name;
@@ -3929,7 +3929,7 @@ FileDescriptor* DescriptorPool::NewPlaceholderFileWithMutexHeld(
     mutex_->AssertHeld();
   }
   FileDescriptor* placeholder = tables_->Allocate<FileDescriptor>();
-  memset(placeholder, 0, sizeof(*placeholder));
+  memset(static_cast<void*>(placeholder), 0, sizeof(*placeholder));
 
   placeholder->name_ = tables_->AllocateString(name);
   placeholder->package_ = &internal::GetEmptyString();