Merge #11620

11620: feat: Support locals with multiple declaration sites r=Veykril a=Veykril

Fixes https://github.com/rust-analyzer/rust-analyzer/issues/8860

This implements the first approach of https://github.com/rust-analyzer/rust-analyzer/issues/8860#issuecomment-845461773:
> treat every Local as having potentially many sources?

The way this is written does allow changing to a MultiLocal approach instead though, I'll investigate whether that turns out to be better or not.

Co-authored-by: Lukas Wirth <lukastw97@gmail.com>