THRIFT-5389: Fix const generation for optional fields

Client: go

The current compiler will generate uncompilable code when we use
optional enum and/or typedef'd types in a thrift constant.

This fixes the issue, also adds a test for that.
diff --git a/lib/go/test/ConstOptionalFieldImport.thrift b/lib/go/test/ConstOptionalFieldImport.thrift
new file mode 100644
index 0000000..a271be1
--- /dev/null
+++ b/lib/go/test/ConstOptionalFieldImport.thrift
@@ -0,0 +1,36 @@
+/*
+ * 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 go constoptionalfielda
+
+enum Foo {
+  One = 1,
+  Two = 2,
+}
+
+typedef Foo TypedefAFoo
+
+typedef bool TypedefABool
+typedef i8 TypedefAI8
+typedef i16 TypedefAI16
+typedef i32 TypedefAI32
+typedef i64 TypedefAI64
+typedef double TypedefADouble
+typedef string TypedefAString
+typedef binary TypedefABinary