Port #[used] to new attribute parsing infrastructure

Signed-off-by: Jonathan Brouwer <jonathantbrouwer@gmail.com>

compiler/rustc_codegen_ssa/src/errors.rs

@@ -726,13 +726,6 @@ pub struct UnknownArchiveKind<'a> {
     pub kind: &'a str,
 }
 
-#[derive(Diagnostic)]
-#[diag(codegen_ssa_expected_used_symbol)]
-pub(crate) struct ExpectedUsedSymbol {
-    #[primary_span]
-    pub span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag(codegen_ssa_multiple_main_functions)]
 #[help]