Support default field values

[Nadrieril]

Rust should soon support the following syntax (rust-lang/rust#132162):

#[derive(Default)]
struct Pet {
    name: Option<String>, // impl Default for Pet will use Default::default() for name
    age: i128 = 42, // impl Default for Pet will use the literal 42 for age
}

This will require special handling as such fields are not desugared to anything else in the THIR.