Auto merge of #91957 - nnethercote:rm-SymbolStr, r=oli-obk

Remove `SymbolStr`

This was originally proposed in https://github.com/rust-lang/rust/pull/74554#discussion_r466203544. As well as removing the icky `SymbolStr` type, it allows the removal of a lot of `&` and `*` occurrences.

Best reviewed one commit at a time.

r? `@oli-obk`

compiler/rustc_resolve/src/lib.rs

@@ -3481,7 +3481,7 @@ fn names_to_string(names: &[Symbol]) -> String {
         if Ident::with_dummy_span(*name).is_raw_guess() {
             result.push_str("r#");
         }
-        result.push_str(&name.as_str());
+        result.push_str(name.as_str());
     }
     result
 }