UPDATE - rename SessionSubdiagnostic macro to Subdiagnostic

Also renames:
- sym::AddSubdiagnostic to sym:: Subdiagnostic
- rustc_diagnostic_item = "AddSubdiagnostic" to rustc_diagnostic_item = "Subdiagnostic"

compiler/rustc_lint/src/errors.rs

@@ -1,5 +1,5 @@
 use rustc_errors::{fluent, AddToDiagnostic, IntoDiagnostic, ErrorGuaranteed, Handler};
-use rustc_macros::{Diagnostic, SessionSubdiagnostic};
+use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_session::lint::Level;
 use rustc_span::{Span, Symbol};
 
@@ -51,7 +51,7 @@ pub struct MalformedAttribute {
     pub sub: MalformedAttributeSub,
 }
 
-#[derive(SessionSubdiagnostic)]
+#[derive(Subdiagnostic)]
 pub enum MalformedAttributeSub {
     #[label(lint::bad_attribute_argument)]
     BadAttributeArgument(#[primary_span] Span),

compiler/rustc_lint/src/internal.rs

@@ -404,7 +404,7 @@ impl LateLintPass<'_> for Diagnostics {
                 let Impl { of_trait: Some(of_trait), .. } = impl_ &&
                 let Some(def_id) = of_trait.trait_def_id() &&
                 let Some(name) = cx.tcx.get_diagnostic_name(def_id) &&
-                matches!(name, sym::Diagnostic | sym::AddSubdiagnostic | sym::DecorateLint)
+                matches!(name, sym::Diagnostic | sym::Subdiagnostic | sym::DecorateLint)
             {
                 found_impl = true;
                 break;