GenFacades fails to generate not supported assembly for reference source that contains throw null expression body

• This issue is blocking
• This issue is causing unreasonable pain

GenFacades not supported assembly fails for reference source files that use the => throw null; syntax instead of a full method body.

C:\git\runtime5\artifacts\obj\System.Net.Quic\net6.0-Browser-Debug\System.Net.Quic.notsupported.cs(17,9): error CS8057: Block bodies and expression bodies cannot both be provided. [C:\git\runtime5\src\libraries\System.Net.Quic\src\System.Net.Quic.csproj]

        public QuicListener(IPEndPoint listenEndPoint, System.Net.Security.SslServerAuthenticationOptions sslServerAuthenticationOptions) { throw new System.PlatformNotSupportedException(System.SR.SystemNetQuic_PlatformNotSupported);  }
=> throw null;

cc @Anipik @ericstj

[ericstj]

@safern is currently investigating. We have discussed with folks from Roslyn in the past and no tool existed but the closest one was the functionality that backed go-to definition. We indeed want Roslyn APIs to back as much of the GenAPI stack as possible so we get new language features automatically. We also desire to publicly ship the tool and there is likely a better place for it to live than arcade or runtime.

[Anipik]

We can use https://apisof.net/catalog/Microsoft.CodeAnalysis.CSharp.Syntax.MethodDeclarationSyntax.ExpressionBody to differentiate between the 2 cases and fix the error.