CS warnings appear in Resources.Designer.cs when a resource file excerpt line starts with `/`

Due to a bug in CodeDom's generation of C# code comments that the StronglyTypedResourceBuilder class in this repo does nothing to avoid, any project with `true` that embeds a file that contains a line that starts with `/` gets invalid C# documentation syntax in .Designer.cs, resulting in warnings:


I asked first at https://github.com/dotnet/roslyn/issues/53729 in case the compiler team thought that documentation warnings should be skipped in generated code, and they decided that the current behavior is by design.

### Steps to Reproduce

Full repro: [Repro.zip](https://github.com/dotnet/roslyn/files/6556702/Repro.zip)

Resource file being included (real-life example was a SQL file that was truncated by the resource code generator when included in the XML comment):

```

/
```

Problematic excerpt from Resources.Designer.cs:

```cs
///
/// Looks up a localized string similar to
////.
///
internal static string SomeResourceFile {
get {
return ResourceManager.GetString("SomeResourceFile", resourceCulture);
}
}
```

Project file:

```xml

net5.0
true

True
True
Resources.resx

ResXFileCodeGenerator
Resources.Designer.cs

```

Specific errors:




### Analysis

This is where CodeDom creates the invalid `////` XML doc comment line: https://github.com/dotnet/runtime/blob/v5.0.6/src/libraries/System.CodeDom/src/Microsoft/CSharp/CSharpCodeGenerator.cs#L879
.NET Framework's CodeDom has the same bug: https://referencesource.microsoft.com/#System/compmod/microsoft/csharp/csharpcodeprovider.cs,e0b125d92a26ca23

On that line it outputs `///` because of `string commentLineStart = e.DocComment ? "///" : "//";` outside that loop. Then the next character of `value` that will be written after that is a single `/`. This causes invalid C# any time a forward slash follows a newline in `CodeComment.Text`.

There does not appear to be a way to work around this by instructing the Resources.Designer.cs generator to stop putting file content in XML comments: https://github.com/dotnet/msbuild/blob/v16.10.0/src/Tasks/system.design/stronglytypedresourcebuilder.cs#L562-L593

A workaround is to put this in .editorconfig:

```ini
[*.Designer.cs] # Workaround for https://github.com/dotnet/roslyn/issues/53729
dotnet_diagnostic.CS1570.severity = none
dotnet_diagnostic.CS1587.severity = none
```

### Versions & Configurations

Visual Studio 16.10.0

This also afflicts the StronglyTypedResourceBuilder in .NET Framework's Microsoft.Build.Tasks.v4.0.dll assembly, file version 4.8.4084.0.