Make unexpected always "return" PResult<()> & add unexpected_any

This prevents breakage when `?` no longer skews inference.

compiler/rustc_builtin_macros/src/asm.rs

@@ -189,7 +189,7 @@ pub fn parse_asm_args<'a>(
             args.templates.push(template);
             continue;
         } else {
-            return p.unexpected();
+            p.unexpected_any()?
         };
 
         allow_templates = false;