Implement DynamicValue::Reader::as<DynamicValue>()

This is required to allow the compiler to successfully instantiate a
JsonCodec::Handler<DynamicValue, Style::POINTER> using Handler's default
partial specialization for Style::POINTER. Such an instantiation is
currently only used in dead code that clang and gcc eliminate but MSVC
does not, which is why the problem was not caught until now.

Also implemented DynamicValue::Builder::as<DynamicValue>() to preserve
symmetry.

c++/src/capnp/dynamic.h

@@ -634,6 +634,7 @@ public:
   // - DynamicEnum:  Returns the corresponding type.
   // - DynamicStruct, DynamicList:  Returns the corresponding Reader.
   // - Any capability type, including DynamicCapability:  Returns the corresponding Client.
+  // - DynamicValue:  Returns an identical Reader. Useful to avoid special-casing in generic code.
   //   (TODO(perf):  On GCC 4.8 / Clang 3.3, provide rvalue-qualified version that avoids
   //   refcounting.)
   //
@@ -1408,6 +1409,19 @@ struct DynamicValue::Builder::AsImpl<T, Kind::INTERFACE> {
   }
 };
 
+template <>
+struct DynamicValue::Reader::AsImpl<DynamicValue> {
+  static DynamicValue::Reader apply(const Reader& reader) {
+    return reader;
+  }
+};
+template <>
+struct DynamicValue::Builder::AsImpl<DynamicValue> {
+  static DynamicValue::Builder apply(Builder& builder) {
+    return builder;
+  }
+};
+
 inline DynamicValue::Pipeline::Pipeline(std::nullptr_t n): type(UNKNOWN) {}
 inline DynamicValue::Pipeline::Pipeline(DynamicStruct::Pipeline&& value)
     : type(STRUCT), structValue(kj::mv(value)) {}