Reduce the number of members on ModelMetadataIdentity

[pranavkm]

ModelMetadataIdentity is a discriminated union. We recently introduced an additional member on this type,
but are concerned about the performance implication of doing this. This change uses a discriminated field
to reset the member count to 3.0.

[mkArtakMSFT]

Thanks Pranav!

[mkArtakMSFT]

@pranavkm I don't think this is transient:

##[error]src\Mvc\Mvc.Abstractions\ref\Microsoft.AspNetCore.Mvc.Abstractions.netcoreapp.cs(,): error : Generated code is not up to date in src/Mvc/Mvc.Abstractions/ref/Microsoft.AspNetCore.Mvc.Abstractions.netcoreapp.cs. You might need to regenerate the reference assemblies or project list (see docs/ReferenceAssemblies.md and docs/ReferenceResolution.md)
error : Generated code is not up to date in src/Mvc/Mvc.Abstractions/ref/Microsoft.AspNetCore.Mvc.Abstractions.netcoreapp.cs. You might need to regenerate the reference assemblies or project list (see docs/ReferenceAssemblies.md and docs/ReferenceResolution.md) [F:\workspace\_work\1\s\src\Mvc\Mvc.Abstractions\ref\Microsoft.AspNetCore.Mvc.Abstractions.netcoreapp.cs]
diff --git a/src/Mvc/Mvc.Abstractions/ref/Microsoft.AspNetCore.Mvc.Abstractions.netcoreapp.cs b/src/Mvc/Mvc.Abstractions/ref/Microsoft.AspNetCore.Mvc.Abstractions.netcoreapp.cs
index e41015f9e9..003cfa201b 100644
--- a/src/Mvc/Mvc.Abstractions/ref/Microsoft.AspNetCore.Mvc.Abstractions.netcoreapp.cs
+++ b/src/Mvc/Mvc.Abstractions/ref/Microsoft.AspNetCore.Mvc.Abstractions.netcoreapp.cs
@@ -882,8 +882,8 @@ namespace Microsoft.AspNetCore.Mvc.ModelBinding.Metadata
         public Microsoft.AspNetCore.Mvc.ModelBinding.Metadata.ModelMetadataKind MetadataKind { get { throw null; } }
         public System.Type ModelType { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
         public string Name { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
-        public System.Reflection.ParameterInfo ParameterInfo { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
-        public System.Reflection.PropertyInfo PropertyInfo { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
+        public System.Reflection.ParameterInfo ParameterInfo { get { throw null; } }
+        public System.Reflection.PropertyInfo PropertyInfo { get { throw null; } }
         public bool Equals(Microsoft.AspNetCore.Mvc.ModelBinding.Metadata.ModelMetadataIdentity other) { throw null; }
         public override bool Equals(object obj) { throw null; }
         public static Microsoft.AspNetCore.Mvc.ModelBinding.Metadata.ModelMetadataIdentity ForParameter(System.Reflection.ParameterInfo parameter) { throw null; }

[pranavkm]

Yup. WIll fix