Rollup merge of #122780 - GuillaumeGomez:rename-hir-local, r=oli-obk

Rename `hir::Local` into `hir::LetStmt`

Follow-up of #122776.

As discussed on [zulip](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Improve.20naming.20of.20.60ExprKind.3A.3ALet.60.3F).

I made this change into a separate PR because I'm less sure about this change as is. For example, we have `visit_local` and `LocalSource` items. Is it fine to keep these two as is (I supposed it is but I prefer to ask) or not? Having `Node::Local(LetStmt)` makes things more explicit but is it going too far?

r? ```@oli-obk```

Author: matthiaskrgr