normalize use of backticks in compiler messages for libsyntax/feature_gate

https://github.com/rust-lang/rust/issues/60532

src/test/ui/feature-gates/feature-gate-alloc-error-handler.rs

@@ -5,7 +5,7 @@
 
 use core::alloc::Layout;
 
-#[alloc_error_handler] //~ ERROR #[alloc_error_handler] is an unstable feature
+#[alloc_error_handler] //~ ERROR `#[alloc_error_handler]` is an unstable feature
 fn oom(info: Layout) -> ! {
     loop {}
 }