Use MIR body to identify more "default equivalent" calls for derivable_impls

[estebank]

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

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

struct R {
    s: S,
}

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

the derivable_impls lint will now trigger.

changelog: [derivable_impls]: detect when a Default impl is using the same fn call that that type's Default::default calls

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[samueltardieu]

Very interesting.

In particular, your change also affects mem_replace_with_default, see the lintcheck run. If those suggestions are valid (and it looks like they are), you should add tests to tests/ui/mem_replace.rs as well to reflect this and make sure we don't regress later.