Make the modifier list a collection

Author: tbkka

CodeGeneration/Sources/SyntaxSupport/DeclNodes.swift

@@ -98,10 +98,9 @@ public let DECL_NODES: [Node] = [
       ),
       Child(
         name: "modifiers",
-        kind: .node(kind: .declModifierList),
+        kind: .collection(kind: .declModifierList, collectionElementName: "Modifier", defaultsToEmpty: true),
         nameForDiagnostics: "modifiers",
-        isOptional: true
-      ),
+        documentation: "Modifiers like `mutating` or `yielding` that affect the accessor declaration."      ),
       Child(
         name: "accessorSpecifier",
         kind: .token(choices: [

Sources/SwiftParser/Declarations.swift

@@ -1658,15 +1658,13 @@ extension Parser {
 
   struct AccessorIntroducer {
     var attributes: RawAttributeListSyntax
-    var modifiers: RawDeclModifierListSyntax?
+    var modifiers: RawDeclModifierListSyntax
     var kind: AccessorDeclSyntax.AccessorSpecifierOptions
     var unexpectedBeforeToken: RawUnexpectedNodesSyntax?
     var token: RawTokenSyntax
   }
 
-  mutating func parseAccessorModifierList(count: Int) -> RawDeclModifierListSyntax? {
-    if count == 0 { return nil }
-
+  mutating func parseAccessorModifierList(count: Int) -> RawDeclModifierListSyntax {
     var elements = [RawDeclModifierSyntax]()
 
     for _ in 0..<count {