Add __shared to SyntaxArena paramenter for initializers

To avoid unnecessary ref-counting traffic.

Author: rintaro

Sources/SwiftSyntax/RawSyntaxNodeProtocol.swift

@@ -112,7 +112,7 @@ public struct RawTokenSyntax: RawSyntaxNodeProtocol {
     kind: RawTokenKind,
     wholeText: SyntaxText,
     textRange: Range<SyntaxText.Index>,
-    arena: SyntaxArena
+    arena: __shared SyntaxArena
   ) {
     assert(arena.contains(text: wholeText),
            "token text must be managed by the arena")
@@ -128,7 +128,7 @@ public struct RawTokenSyntax: RawSyntaxNodeProtocol {
     text: SyntaxText,
     leadingTriviaPieces: [RawTriviaPiece],
     trailingTriviaPieces: [RawTriviaPiece],
-    arena: SyntaxArena
+    arena: __shared SyntaxArena
   ) {
     assert(arena.contains(text: text), "token text must be managed by the arena")
     let totalTriviaCount = leadingTriviaPieces.count + trailingTriviaPieces.count
@@ -153,7 +153,7 @@ public struct RawTokenSyntax: RawSyntaxNodeProtocol {
   }
 
   /// Creates a missing `TokenSyntax` with the specified kind.
-  public init(missing kind: RawTokenKind, arena: SyntaxArena) {
+  public init(missing kind: RawTokenKind, arena: __shared SyntaxArena) {
     self.init(
       kind: kind, text: "", leadingTriviaPieces: [], trailingTriviaPieces: [],
       arena: arena)

Sources/SwiftSyntax/RawSyntaxNodes.swift.gyb

@@ -59,7 +59,7 @@ public struct Raw${node.name}: Raw${node.name if node.is_base() else node.base_t
 %   end
 %
 %   if node.is_syntax_collection():
-  public init(elements: [Raw${node.collection_element_type}], arena: SyntaxArena) {
+  public init(elements: [Raw${node.collection_element_type}], arena: __shared SyntaxArena) {
     let raw = RawSyntax.makeLayout(
       kind: .${node.swift_syntax_kind}, uninitializedCount: elements.count, arena: arena) { layout in
       guard var ptr = layout.baseAddress else { return }
@@ -85,7 +85,7 @@ public struct Raw${node.name}: Raw${node.name if node.is_base() else node.base_t
 %       param_default = " = nil" if child.is_unexpected_nodes() else ""
     ${param_label}${child.swift_name}: ${param_type}${param_default},
 %     end
-    arena: SyntaxArena
+    arena: __shared SyntaxArena
   ) {
 %     if node.children:
     let raw = RawSyntax.makeLayout(