RDL property type loss

[kennykerr]

A similarly structured RDL surfaces a genuine bug:

#[winrt]
mod Test {
    interface ITest {
        #[get]
        foo: u32;
        #[set]
        foo: String; // <-- foo is of type String, not u32
    }
}

Running this RDL through the same Reader/Writer-cycle produces the same RDL output as the previous test (foo: u32;), dropping the type information for the #[set]-attributed property of the same symbol foo. This needs to be addressed.

Originally posted by @tim-weis in #4143 (comment)