THRIFT-5179: Fix generated code for struct's named "a" or "b"

Client: cpp
diff --git a/test/SpecificNameTest.thrift b/test/SpecificNameTest.thrift
new file mode 100644
index 0000000..165fc31
--- /dev/null
+++ b/test/SpecificNameTest.thrift
@@ -0,0 +1,35 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+namespace cpp test.specificname
+
+struct a {
+  1: i32 number,
+  2: string message,
+}
+
+struct b {
+  1: i32 number,
+  2: string message,
+}
+
+service EchoService {
+  void echoA(1: a arg),
+  void echoB(2: b arg),
+}