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

compiler/rustc_borrowck/src/lib.rs

@@ -690,6 +690,8 @@ impl<'cx, 'tcx> rustc_mir_dataflow::ResultsVisitor<'cx, 'tcx> for MirBorrowckCtx
             }
             // Only relevant for mir typeck
             StatementKind::AscribeUserType(..)
+            // Only relevant for unsafeck
+            | StatementKind::PlaceMention(..)
             // Doesn't have any language semantics
             | StatementKind::Coverage(..)
             // These do not actually affect borrowck