UPDATE - rename SessionSubdiagnostic macro to Subdiagnostic

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

compiler/rustc_middle/src/error.rs

@@ -25,7 +25,7 @@ pub struct OpaqueHiddenTypeMismatch<'tcx> {
     pub sub: TypeMismatchReason,
 }
 
-#[derive(SessionSubdiagnostic)]
+#[derive(Subdiagnostic)]
 pub enum TypeMismatchReason {
     #[label(middle::conflict_types)]
     ConflictType {