Fix undefined symbol error around SharedCtor()

Apparently, protocolbuffers#8532 was incorrect if applied to 3.17.x branch.

3.17.x changed code generation to mark `SharedCtor()` and `SharedDtor()` as inline in .pb.cc.

It looks like we have a compile-time undefined behavior in C++ now. As cppreference.com [says](https://en.cppreference.com/w/cpp/language/inline):

_The definition of an inline function <...> must be reachable in the translation unit where it is accessed (not necessarily before the point of access)._

As protobuf allows custom plugins to generate custom code, there is no limitation on where SharedCtor couble be possible referenced from. In our case we have SharedCtor invoked from corresponding `.pb.h` code, thus triggering:
```
ld: error: undefined symbol: package::Message::SharedCtor()`
>>> referenced by file.pb.h:$$$$
```

If this patch is not applicable, I can take a look into changing the code generation, but doing this will be harder then removing _inline_.

src/google/protobuf/compiler/cpp/cpp_message.cc

@@ -2381,7 +2381,7 @@ std::pair<size_t, size_t> MessageGenerator::GenerateOffsets(
 void MessageGenerator::GenerateSharedConstructorCode(io::Printer* printer) {
   Formatter format(printer, variables_);
 
-  format("inline void $classname$::SharedCtor() {\n");
+  format("void $classname$::SharedCtor() {\n");
 
   std::vector<bool> processed(optimized_order_.size(), false);
   GenerateConstructorBody(printer, processed, false);