optimize: generate the go type for const

[luryson]

Change-Id: I53ea6980c732f71c6fd27b527e24155ce54f8666

Description

add const type

Motivation and Context

const definition is generated without specified type.
It's confusing for client users to find optional values of that type. In my opinion, when typedef types and corresponding values are defined, it is to inform users of the available values of that type.

#174

Related Issue

[CLAassistant]

All committers have signed the CLA.

[HeyJavaBean]

unit test挂了，可以对应修改下

[luryson]

@HeyJavaBean 本地试了多次，好像不是个必现的错误，我先看看能不能浮现出来

[HeyJavaBean]

看着是其他模块的报错，我来查吧。如果和这个pr没关系的话我就合了

[luryson]

调了很多次，捕获到一次这个错误：

=== RUN   TestSimpleGrammar
person_struct_info:failed to parse option:field not exist:name:'123'}, innerStruct:{email
    option_test.go:625: assertion failed
--- FAIL: TestSimpleGrammar (0.01s)

看了一下 idl，有 typedef ，不确定是否有关系

[HeyJavaBean]

没有关系