typing.TYPE_CHECKING not true during dev

Seems like typing.TYPE_CHECKING is not true during dev.
e.g.:
```py
from typing import TYPE_CHECKING:
if TYPE_CHECKING:
from somewhere import MyClass

async def myFunction(x:str, a:int, c:"MyClass"):
c.someva # no auto-complete or suggestion here

```

if line 2 is `if TYPE_CHECKING or True`
code will be suggested and autocompleted.
based on typing.py L1444:
```py
# Constant that's True when type checking, but False here.
TYPE_CHECKING = False
```
OS Ubuntu 18.04.2 LTS
Python 3.7.1

Am i missing something, or wrong configuration?

首先，使用 TYPE_CHECKING 示例在 Ubuntu 18.04.2 上通过 Python 3.7.1 重现报告的行为，并将其与 `typing.py` L1444 处的参考进行比较。跟踪 IDE 自动补全或语言服务器的入口点；当由 TYPE_CHECKING 保护的导入在不改变条件的情况下提供预期的建议时，即表示完成。

由索引模型根据 Issue 内容生成。