Use MIR body to identify more "default equivalent" calls

When looking for `Default` impls that could be derived, we look at the
body of their `fn default()` and if it is an fn call or literal we check
if they are equivalent to what `#[derive(Default)]` would have used.

Now, when checking those fn calls in the `fn default()` body, we also
compare against the corresponding type's `Default::default` body to see
if our call is equivalent to that one.

For example, given

```rust
struct S;

impl S {
    fn new() -> S { S }
}

impl Default for S {
    fn default() -> S { S::new() }
}
```

`<S as Default>::default()` and `S::new()` are considered equivalent.
Given that, if the user also writes

```rust
struct R {
    s: S,
}

impl Default for R {
    fn default() -> R {
        R { s: S::new() }
    }
}
```

the `derivable_impls` lint will now trigger.

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