Introduce a no-op `PlaceMention` statement for `let _ =`.
Fixes https://github.com/rust-lang/rust/issues/54003
Fixes https://github.com/rust-lang/rust/issues/80059
Split from https://github.com/rust-lang/rust/pull/101500
This PR introduces a new `PlaceMention` statement dedicated to matches that neither introduce bindings nor ascribe types. Without this, all traces of the match would vanish from MIR, making it impossible to diagnose unsafety or use in #101500.
This allows to mark `let _ = <unsafe union access or dereference>` as requiring an unsafe block.
Nominating for lang team, as this introduces an extra error.
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