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

Author: cjgillot

clippy_utils/src/qualify_min_const_fn.rs

@@ -241,6 +241,7 @@ fn check_statement<'tcx>(
         | StatementKind::StorageDead(_)
         | StatementKind::Retag { .. }
         | StatementKind::AscribeUserType(..)
+        | StatementKind::PlaceMention(..)
         | StatementKind::Coverage(..)
         | StatementKind::ConstEvalCounter
         | StatementKind::Nop => Ok(()),