[c#] Generated code for non-alphanumeric enum names is invalid

[saquibmian]

What version of protobuf and what language are you using?
Version: v28.3
Language: C#

What operating system (Linux, Windows, ...) and version?
macOS

What runtime / compiler are you using (e.g., python version or gcc version)
protoc

What did you do?
Steps to reproduce the behavior:

1. Write foo.proto with the following content.

   syntax = "proto3";
   
   package foo.v1;
   
   enum Foo {
     _ = 0;
   }

2. Run protoc -I . --csharp_out=. foo.proto

What did you expect to see
Foo.cs should contain the following definition:

public enum Foo {
  [pbr::OriginalName("_")]  _ = 0,
}

What did you see instead?
Foo.cs contains the following definition, which does not compile (missing enum name):

public enum Foo {
  [pbr::OriginalName("_")]  = 0,
}

Anything else we should know about your project / environment
Nothing else.

[jskeet]

I agree we shouldn't generate invalid code. Whether just _ is the right name to generate is up for discussion, given that _ has a "somewhat" special meaning in C#. (For an enum value that make be okay - for a field it could be pretty confusing. It's worth thinking about how it might be used in all contexts rather than just enums.)

I think it's unlikely that we're going to get to this corner case any time soon - but if you have the time to create a PR to change the C# generator code to fix this, I'd happily review it.

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