Use ZST for fmt unsafety

This allows the format_args! macro to keep the pre-expansion code out of
the unsafe block without doing gymnastics with nested `match`
expressions. This reduces codegen.

compiler/rustc_span/src/symbol.rs

@@ -253,6 +253,7 @@ symbols! {
         TyCtxt,
         TyKind,
         Unknown,
+        UnsafeArg,
         Vec,
         Yield,
         _DECLS,