Create anon const defs in DefCollector unconditionally

[BoxyUwU]

Fixes #133064

Macros in statement position can expand to nothing which fundamentally breaks any attempt to not create DefIds that don't need to be created. When a macro expands to nothing we never call visit_expr on its expanded results so never create the DefId for the anon const which leads to ICEs. So... just completely give up on this and make def encoding deal with having defids exist that shouldn't.

Alternatively we could make macro!{} never expand to nothing and always atleast be an empty statement, this would be a breaking change due to:

macro_rules! empty {
    () => {};
}

fn bar() -> usize {
    {
        1
    }
    empty! {}
}

compiling on stable

r? @compiler-errors

[BoxyUwU]

Should also add the following test:

struct Foo<const N: usize>;

macro_rules! expands_to_n {
    () => {
        N
    };
}

macro_rules! empty {
    () => {};
}

const N: usize = 10;

type A = Foo<
    {
        empty! {}
        expands_to_n! {}
    },
>;

which demonstrates why we cant simply special case a fully expanded const argument being {}

[BoxyUwU]

Actually going to close this until lcnr has had a chance to play around with this since they said they wanted to take a look and see if they could find a clean way to make this work