Use structured suggestion for braceless unicode escape squence

src/test/ui/parser/issue-23620-invalid-escapes.rs

@@ -35,4 +35,7 @@ fn main() {
     //~^ ERROR invalid character in numeric character escape:
     //~^^ ERROR form of character escape may only be used with characters in the range [\x00-\x7f]
     //~^^^ ERROR incorrect unicode escape sequence
+
+    let _ = "\u8f";
+    //~^ ERROR incorrect unicode escape sequence
 }

src/test/ui/parser/issue-23620-invalid-escapes.stderr

@@ -118,5 +118,13 @@ help: format of unicode escape sequences is `/u{…}`
 LL |     let _ = "/xf /u";
    |                  ^^
 
-error: aborting due to 17 previous errors
+error: incorrect unicode escape sequence
+  --> $DIR/issue-23620-invalid-escapes.rs:39:14
+   |
+LL |     let _ = "/u8f";
+   |              ^^--
+   |              |
+   |              help: format of unicode escape sequences uses braces: `/u{8f}`
+
+error: aborting due to 18 previous errors