Introduce a no-op PlaceMention statement for let _ =.

compiler/rustc_mir_dataflow/src/impls/liveness.rs

@@ -263,6 +263,7 @@ impl<'a, 'tcx> Analysis<'tcx> for MaybeTransitiveLiveLocals<'a> {
             | StatementKind::StorageDead(_)
             | StatementKind::Retag(..)
             | StatementKind::AscribeUserType(..)
+            | StatementKind::PlaceMention(..)
             | StatementKind::Coverage(..)
             | StatementKind::Intrinsic(..)
             | StatementKind::ConstEvalCounter

compiler/rustc_mir_dataflow/src/impls/storage_liveness.rs

@@ -139,6 +139,7 @@ impl<'mir, 'tcx> crate::GenKillAnalysis<'tcx> for MaybeRequiresStorage<'mir, 'tc
             // Nothing to do for these. Match exhaustively so this fails to compile when new
             // variants are added.
             StatementKind::AscribeUserType(..)
+            | StatementKind::PlaceMention(..)
             | StatementKind::Coverage(..)
             | StatementKind::FakeRead(..)
             | StatementKind::ConstEvalCounter