Don't allow new const panic through format flattening.

panic!("a {}", "b") is still not allowed in const,
even if the hir flattens to panic!("a b").

compiler/rustc_span/src/symbol.rs

@@ -984,6 +984,7 @@ symbols! {
         never_type_fallback,
         new,
         new_binary,
+        new_const,
         new_debug,
         new_display,
         new_lower_exp,