Allow block_in_if_{stmt,expr} in external macro
I found this by running `cargo fix --clippy` on quite a big codebase.
You could refactor this assert to
```rust
let block_expr = _;
assert!(block_expr);
```
but,
1. it doesn't increase the readability IMO
2. That isn't possible in a `debug_assert!`
I'm not sure though, if we should allow this for macros in general or just for external macros.
changelog: Allow `block_in_if_{stmt,expr}` in external macros
8af4e09605