Avoid marking ThenStmtSyntax experimental for now

Author: hamishknight

CodeGeneration/Sources/SyntaxSupport/KeywordSpec.swift

@@ -646,7 +646,7 @@ public enum Keyword: CaseIterable {
     case .target:
       return KeywordSpec("target")
     case .then:
-      return KeywordSpec("then", isExperimental: true)
+      return KeywordSpec("then")
     case .throw:
       return KeywordSpec("throw", isLexerClassified: true)
     case .throws:

CodeGeneration/Sources/SyntaxSupport/StmtNodes.swift

@@ -617,7 +617,8 @@ public let STMT_NODES: [Node] = [
   Node(
     kind: .thenStmt,
     base: .stmt,
-    isExperimental: true,
+    // FIXME: This should be marked experimental.
+    isExperimental: false,
     nameForDiagnostics: "'then' statement",
     documentation: """
       A statement used to indicate the produced value from an if/switch