Add "unsafe" keyword to the for..in loop

Author: DougGregor

CodeGeneration/Sources/SyntaxSupport/StmtNodes.swift

@@ -284,6 +284,12 @@ public let STMT_NODES: [Node] = [
         kind: .token(choices: [.keyword(.await)]),
         isOptional: true
       ),
+      Child(
+        name: "unsafeKeyword",
+        kind: .token(choices: [.keyword(.unsafe)]),
+        experimentalFeature: .unsafeExpression,
+        isOptional: true
+      ),
       Child(
         name: "caseKeyword",
         kind: .token(choices: [.keyword(.case)]),