Fix invalid special casing of the unreachable! macro

src/test/ui/macros/unreachable-format-arg.rs

@@ -0,0 +1,13 @@
+// run-fail
+// ignore-emscripten no processes
+
+// revisions: edition_2015 edition_2021
+// [edition_2015]edition:2015
+// [edition_2021]edition:2021
+// [edition_2015]error-pattern:internal error: entered unreachable code: x is {x}
+// [edition_2021]error-pattern:internal error: entered unreachable code: x is 5
+
+fn main() {
+    let x = 5;
+    unreachable!("x is {x}");
+}