Expand assert!(expr) to panic() function instead of panic!() macro.

The panic message might contain braces which should never be
interpreted as format placeholders, which panic!() will do in a future
edition.

compiler/rustc_span/src/symbol.rs

@@ -791,6 +791,7 @@ symbols! {
         panic_runtime,
         panic_str,
         panic_unwind,
+        panicking,
         param_attrs,
         parent_trait,
         partial_cmp,