Suggest br if the unknown string prefix rb is found

src/test/ui/suggestions/raw-byte-string-prefix.rs

@@ -0,0 +1,10 @@
+// `br` and `rb` are easy to confuse; check that we issue a suggestion to help.
+
+// edition:2021
+
+fn main() {
+    rb"abc";
+    //~^ ERROR: prefix `rb` is unknown
+    //~| HELP: use `br` for a raw byte string
+    //~| ERROR: expected one of
+}

src/test/ui/suggestions/raw-byte-string-prefix.stderr

@@ -0,0 +1,20 @@
+error: prefix `rb` is unknown
+  --> $DIR/raw-byte-string-prefix.rs:6:5
+   |
+LL |     rb"abc";
+   |     ^^ unknown prefix
+   |
+   = note: prefixed identifiers and literals are reserved since Rust 2021
+help: use `br` for a raw byte string
+   |
+LL |     br"abc";
+   |     ^^
+
+error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `"abc"`
+  --> $DIR/raw-byte-string-prefix.rs:6:7
+   |
+LL |     rb"abc";
+   |       ^^^^^ expected one of 8 possible tokens
+
+error: aborting due to 2 previous errors
+