Make yielding a plain decl modifier instead of a new node type

Author: tbkka

CodeGeneration/Sources/SyntaxSupport/DeclNodes.swift

@@ -180,12 +180,6 @@ public let DECL_NODES: [Node] = [
     ]
   ),
 
-  Node(
-    kind: .accessorYieldingModifier,
-    base: .syntax,
-    nameForDiagnostics: nil,
-  ),
-
   Node(
     kind: .actorDecl,
     base: .decl,
@@ -501,6 +495,7 @@ public let DECL_NODES: [Node] = [
           .keyword(.unowned),
           .keyword(.weak),
           .keyword(.sending),
+          .keyword(.yielding),
         ])
       ),
       Child(