Completions in macros should be aware of completing multiple idents

When typing inside a macro we should consider all downmapped positions of the ident we are currently typing out instead of just the first one, that way we get more completions that might make sense.

Example given the following macro:

macro_rules! tuple_to_record_closure {
    ($ident:ident { $( $field:ident ),+ $(,)? }) => {
        |($( $field ),+)| $ident { $( $field ),+ }
    };
}
struct Foo { foo: () }
fn main() {
    tuple_to_record_closure! {
        Foo {
            f$0
        }
    }
}

Typing at the given position should offer the completion of the foo field as the ident is downmapped to a field position as its second usage, currently we only consider the parameter position though.

This also has the potential to create a lot of duplicate completions which have to be filtered out, as well as making completions in macros potentially very slow if we were to stupidly repeat completions for each downmapped token, so this probably requires some architecture changes if we want to pursue this.

cc https://github.com/rust-analyzer/rust-analyzer/issues/11059

Start with the macro example in the issue and trace completion handling for downmapped identifier positions. Compare the current first-position behavior with all downmapped uses, including the field position. Done means the example offers the Foo.foo field completion without duplicate results or impractical slowdown.