Avoid marking ThenStmtSyntax experimental for now

Author: hamishknight

CodeGeneration/Sources/SyntaxSupport/KeywordSpec.swift

@@ -235,7 +235,8 @@ public let KEYWORDS: [KeywordSpec] = [
   KeywordSpec("swift"),
   KeywordSpec("switch", isLexerClassified: true, requiresTrailingSpace: true),
   KeywordSpec("target"),
-  KeywordSpec("then", isExperimental: true),
+  // FIXME: This should be marked experimental.
+  KeywordSpec("then", isExperimental: false),
   KeywordSpec("throw", isLexerClassified: true, requiresTrailingSpace: true),
   KeywordSpec("throws", isLexerClassified: true, requiresTrailingSpace: true),
   KeywordSpec("transpose"),

CodeGeneration/Sources/SyntaxSupport/StmtNodes.swift

@@ -619,7 +619,8 @@ 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