Don't use `maybe_unwrap_block` when checking for macro calls in a block expr
Fixes#131915
Using `maybe_unwrap_block` to determine if we are looking at a `{ mac_call!{} }` will fail sometimes as `mac_call!{}` could be a `StmtKind::MacCall` not a `StmtKind::Expr`. This caused the def collector to think that `{ mac_call!{} }` was a non-trivial const argument and create a definition for it even though it should not.
r? `@compiler-errors` cc `@camelid`
b7dc4813a8