Refresh System.Runtime ref from System.Private.CoreLib in Coreclr

This also includes an update to the genapi tool that includes the
necessary private fields in structs to enable the compiler to
do correctness checks.

See https://github.com/dotnet/corefx/issues/6185.

Author: weshaggard

src/System.Runtime/ref/System.Runtime.Manual.cs

@@ -44,6 +44,6 @@ namespace System.Reflection
     // In the meantime, these will be exposed via wrapper factory methods in System.Private.Reflection.Extensibility.
     public partial struct CustomAttributeNamedArgument
     {
-        internal CustomAttributeNamedArgument(Type attributeType, string memberName, bool isField, CustomAttributeTypedArgument typedValue) { }
+        internal CustomAttributeNamedArgument(Type attributeType, string memberName, bool isField, CustomAttributeTypedArgument typedValue) { throw null; }
     }
 }