Fix lint scoping for let-else.
The scoping for let-else is inconsistent with HIR nesting. This creates cases, in `ui/let-else/let-else-allow-unused.rs` for instance, where an `allow` lint attribute does not apply to the bindings created by `let-else`.
This PR is an attempt to correct this.
As there is no lint that currently relies on this, the test for this behaviour is https://github.com/rust-lang/rust/pull/101500.
cc `@dingxiangfei2009` as you filed https://github.com/rust-lang/rust/pull/101894
1d1f142660